CHAPTER-I

INTRODUCTION, REVIEW OF LITERATURE AND OBJECTIVES

1.1 Introduction

A crystal is a material that is formed by a regular repeated pattern of atoms,

molecules or ions connecting together. In crystals, a collection of atoms called the

unit cell is repeated in exactly the same arrangement over and over throughout the

entire material. Because of this repetitive nature, crystals can take on strange and

interesting looking forms. Generally, crystals form when they undergo a process of

solidification. Under ideal conditions, the result may be a crystal, where all of the

atoms in the solid fit into the same crystal lattice or crystal structure. Sometimes, a

mineral forming in the earth’s crust grows into a particular geometric shape are each

mineral has its own crystal shape and this solid body has a characteristic internal

structure. Some examples of crystals are ice, snow, sugar, metals like gold, silver,

copper, iron and the precious stones like zircon, emerald, topaz, ruby and sapphire etc

[1, 2].

Crystals have fascinated men and women for thousands of years. Naturally

occurring hard gemstone crystals were priced along with gold in antiquity. The

scientific approach to crystal growth was born during the early 17 th century, when

Kepler correlated to morphology and structure, followed by Nicolous Stero who

explained the origin of a variety of external forms exhibited by natural quartz crystals

in terms of different growth rates in different crystallographic directions. The work

carried out during the 19th century, laid a firm foundation for its modern-scientific and

technological developments in crystal growth. The word crystal is from the Greek

word Krystallos, meaning ‘clear ice’. It was first applied to the clear crystals of quartz

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found in the Swiss Alps, because these were thought to be formed from water under

conditions of intense cold. Such crystals actually reflect a highly symmetric and a

periodic internal arrangement of atoms; this is characteristic of all crystalline

materials like metals, alloys, minerals ceramics and some organic materials [3, 4].

Modern technology is largely based on materials such as semiconductors,

superconductors, piezoelectric, ferroelectric, infrared sensitive crystals and nonlinear

optical materials. And efforts are being taken to grow large crystals in short duration

by applying many growth techniques. There is a vast market for solid state devices in

the field of computer, telecommunications etc. Crystals find their applications in solid

state devices, polarizers, radiation detectors, medicine, ultrasonic amplifiers, lasers,

nonlinear optics, piezo-electric, acousto-optic and photosensitive devices.

Triglycine Sulphate (TGS) is one of the most useful ferroelectric material which

finds wide applications as room temperature infrared detectors and in the fabrication

of capacitors, transducers, burglar alarms, medical vidicons and sensors etc [5, 6]. In

this work, it is planned to introduce acids like nitric acid, picric acid, percholoric acid

and salicylic acid as separately as admixtures in TGS crystal to modify its physical

and chemical properties. The aim of this research work is to grow and study the

various properties of nitric acid, picric acid, percholoric acid and salicylic acid

admixtured TGS crystals.

1.2. Classification of crystals

Generally, matter is classified into three states viz. the solid state, the liquid state

and the gaseous state. Matter in the solid state can be classified into crystalline,

amorphous and quasi-crystalline. The crystalline state differs from the amorphous

state in a regular arrangement of the constituent molecules or ions into some fixed and

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rigid pattern known as a lattice. Amorphous solids have incompressibility and rigidity

but they do not have a geometrical regularity or periodicity in the way in which atoms

are arranged in space. Examples for amorphous solids are glass, fused silica, rubber,

polymers etc. In quasi-crystalline state, the atoms are in ordered arrays, but the

patterns they assume are stable and do not recur at regular intervals of distance.

Crystalline materials can be classified into single crystals, poly crystalline materials,

liquid crystals, quasi crystals and nano crystalline materials [7, 8].When the

periodicity of the pattern-unit extends throughout a certain piece of material, it is

called a single crystal. A single crystal is a crystalline solid in which the crystal lattice

of the entire sample is continuous and unbroken to the edges of the sample, with no

grain boundaries. In poly crystalline materials, the periodicity of structure is

interrupted at grain boundaries and the size of the grains in which the structure is

periodic, which may vary from macroscopic dimensions to several angstroms, and if

is equivalent to the combination of a number of single crystals attached together at

some point. Quasi crystals are structural forms that are ordered and non-periodic.

They form patterns that fill the space but lack translational symmetry. Normal crystals

allow only 2, 3, 4 and 6 fold rotational symmetries but quasi crystals display other

rotational folds of symmetries. They were considered to be mathematical artifacts,

known as a periodic tiling, but physical experiments gave evidence of their material

existence.

Nano crystalline materials have grain sizes in the order of 1-100 nm and the

majority of the atoms are located on the surface of the particles. Since the majority of

atoms are in a different environment, the intrinsic properties of nano materials are

different when compared to conventional materials. The above classification of

materials is illustrated in the figure 1.1.

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 Matter

Solids
Fluids

Crystalline Amorphous Quasi crystalline Liquids Gases

Macro (bulk) crystals Micro crystals Nano crystals

Single crystals Poly crystals

1.1.

Figure 1.1: Classification of materials

1.3. Some applications of crystals

It is believed that from the astrological point of view, the crystals of

particular colors play an important role to strengthen the position of the respective

planets in the horoscope. For example, a topaz with yellow tinge is believed to be

useful for strengthening Jupiter. In the metaphysics, however, the applications of

different crystals are found since ages. All modern physical properties have

counterparts in the mental or psychic world; as per beliefs, for example, amplification

in crystals can amplify the body’s energy and thoughts. The beauty of crystals has

always been fascinating, the enchanting colours, the smooth surfaces with scintillating

reflections of light, the definite and varied shapes with sharp edges, the transparency

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of some perfect crystals, all together aroused the aesthetic sense of the early man who

used them as ornaments. Today crystals are the active elements of immense

potentials in science, technology and industry. The crystalline property has

applications in devices such as laser resonators, acoustic modulators, phase decay

plates, polarizers, pyro-electric detectors, piezo-electric devices, crystal X-ray

monochromators, scintillation detectors, holographic devices, membranes of iron

selective electrodes and substrate for thin films. Since crystals of suitable size and

perfection are required for fundamental acquisition and for many applications, crystal

growth is a vital and fundamental part of materials science and engineering. Behind

every new solid state device there stands a single crystal and many new crystals have

to be grown and fabricated in order to assess their device properties. The ever

increasing application of electronics creates an enormous demand for high quality

semiconducting, ferroelectric, piezoelectric, non linear optical and oxide single

crystals. Crystals are used in watches, transistorized radios, computers, laser units and

many other machines. Even nature’s enormous laboratory is no longer able to meet

the demands of developing technology and special factories have appeared where

various crystals ranging in size from very small crystals to large crystals weighing

several kilos are grown.

The requirements of the electronic industry have stimulated many advances

in crystal growth and crystals serve man in many ways and therefore from electronics

to precious gems their unique properties find countless uses. Apart from pure crystals,

mixed and impurity added ones are also involved in many devices. The consistency of

the characteristics of devices fabricated from a crystal depends on the homogeneity

and defect content of the crystals. The interaction of laser light with crystals like

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barium titanate produces a small change in the refractive effect and it is exploited for

recording holograms and the development of phase conjugate optics.

Photonic crystals are attractive optical materials for controlling and manipulating the

flow of light. One dimensional photonic crystal are already in widespread use in the

form of the thin film optics with applications ranging from low and high reflection

coatings on lenses and mirrors to colour changing paints and inks. Higher dimensional

photonic crystals are of great interest for both fundamental and applied research and

the two dimensionally periodic photonic crystals are already available in the form of

photonic crystal fibers which use a nano scale structure to confine light with radically

different characteristics compared to conventional optical fibers for applications in

non linear devices and guiding exotic wavelengths. Attempts are made to replace the

naturally available crystals (gems) by artificially grown crystals and this has been

going on for a long time. Crystals play an important role in the development of high

temperature superconductors and new technological important electro-optical devices

[9, 10].

1.4. Theories of crystal growth

Crystal growth is a complex process in which phase change is important.

Nucleation and growth kinetics, both are the important aspects of such changes. To

explain these aspects, several theories have been proposed by investigators. The

surface energy theory and the diffusion theory give a fair description of the growth

process but all these theories are found to be unsatisfactory in explaining all features

of crystal growth. Gibbs proposed a theory by drawing the analogy between the

growth of water droplet in mist and the growth of a crystal.

Kossel [11] and others analyzed the atomic inhomogeneity of a crystal and

explained the role of step and kink sites on the growth process. This theory became

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popular even though it is not capable to provide a complete explanation for the

continuous growth of a crystal surface. Frank [12] showed that crystal dislocations

were capable of providing the sources of steps required for the continuous growth of a

crystal. A great number of authors have described these theories thoroughly [13-16].

1.4.1 Surface energy theory

Gibbs put forward the surface energy theory on the basis of thermodynamical

treatment of equilibrium states. The growth of a crystal is similar to the formation of

water droplet in mist. The equilibrium state corresponds to total surface energy

becoming minimum for a given volume. Wulff [17] deduced a relation by connecting

the growth rate and the solubility and it is suggested that, when the difference in

solubility is small, the growth is mainly governed by surface energy. Berthound,

Buckley and Valeton [18-20] assailed Curie’s theory on the basis of supersaturation.

They argued that greater supersaturation would cause rapid growth and the crystal

habit ought to be in a spherical shape and this was in contradiction to the observed

facts.

1.4.2 Diffusion theories

Diffusion theories proposed by Noyes, Whitney and Nernst [21, 22] are based

on the following assumptions:

(a) There is a concentration gradient in the vicinity of the surface

(b) The growth process is a reverse process of dissolution

According to them, the amount of solute molecules that gets deposited over

the surface of growing the crystal in the supersaturated solution can be written as

dm/dt= (D/δ) A(c-co)

Where dm is the mass of solute deposited over a crystal surface of area A during

time dt, D is the diffusion coefficient of the solute, c and co are actual and equilibrium

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concentration of the solute and δ the thickness of the stagnant layer adjacent to the

crystal surface. But this theory failed to be consistent with experimental observations.

1.4.3 The surface nucleation models

The surface nucleation model of crystal growth is based on the nucleation

mechanism suggested by Volmer, Kossel, Stranski and others [23-27]. It is a

conventional model of the growth process on the basis of Kossel-Stranski-Volmer

model theory. According to this theory, a crystal has surfaces covered by steps with

terraces between them. These steps possess kinks and there are three types of sites;

terrace, ledge and kink sites. Burton-Cabrera-Frank [28] studied in detail the process

of the advancement of the steps. A step advances by the way of incorporating more

and more atoms at the sites.

1.4.4 Screw dislocation theory

The lack of agreement between the growth rate of crystals by theoretical

calculations and the experimentally observed values leads to the conclusion that the

prediction based on two dimensional nucleation theories may not be the correct

mechanism, responsible for the continuous growth of a crystal surface. It has been

shown by Frank [12] and his collaborators that in some cases the presence of

dislocation may be the controlling factor in the crystal growth. When crystals are

grown, in conditions of low supersaturation, of the order of one percent, it has been

observed that the growth rate is enormously faster than that was calculated for an

ideal crystal. This is substantial disagreement between observation and theory. Frank

explains the actual growth rate in terms of the effect of dislocations on growth.

Frank model of crystal growth is well founded, as there is an excellent agreement

between calculated and observed growth rate, as well as the direct observation of

spiral pattern, the characteristic of this mechanism and it is observed the fact that the

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edge dislocation can also act a as source of step for crystal growth. A general

nucleation at edge dislocation proposed by Frank and Griffin suggested that the

surface stresses provide the extra energy required for the formation of the growth of

nuclei when the dislocation component perpendicular to the surface is absent [29-34].

1.5 Crystal growth methods

Growth of single crystals means careful arrangements of atoms, ions or

molecules in the particular three dimensional orders. It is a control phase-

transformation to ordered solid phase. Crystal growth process must be as close to

equilibrium and steady state process as possible. So both the control of the crystal

growth environment and consideration of growth kinetics are the macroscopic and

sub-microscopic levels that are of vital importance to the success of a crystal growth

experiment.

According to the phase transition, the methods of growing single crystals are

classified as

Solid state growth - Solid  Solid phase transition

Vapour phase growth - Vapour  Solid phase transition

Melt growth - Liquid  Solid phase transition

Solution growth - Liquid  Solid phase transition

There are number of growth techniques under the different phase transition

mentioned above [35-40]. A brief description of the important techniques is disdained

as follows.

1.5.1 Solid state growth

In this technique, the growth of a single crystal takes place from the poly crystalline

mass to a particular material which we intend to grow. Large crystals of many

materials in which mainly metals are grown by this method. The growth will take

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place at low temperature without the presence of additional component which is the

advantageous feature of this technique. But it is difficult to control nucleation as the

growth takes place in the solid and the density of sites of nucleation is high.

1.5.2 Vapour state growth

The vapour growth technique is utilized to grow bulk crystals and for

preparing thin layers on crystals with a high degree of purity. This method is usually

employed to grow crystals of the materials, which do not have a suitable solvent and

sublime before melting at normal pressure. Generally the growth from vapour phase is

divided into two types.

a) Physical Vapour Transport (PVT) method

b) Chemical Vapour Transport (CVT) method

(a) Physical Vapour Transport (PVT) method

The PVT methods are utilized to grow material having satisfactorily high vapour

pressure at attainable temperature. Generally, two types of techniques are employed in

PVT process. They are sublimation-condensation and sputtering. In sublimation

condensation growth, the sublimation of the charge at the hot end of the furnace is

followed by condensation at the cold end [41, 42]. In sputtering process, substances

having the low vapour pressure are used and are best suited for the growth of thin

films. A variety of the crystals have been grown by the PVT method. This method has

also been used extensively for fabricating epitaxial films.

(b) Chemical Vapour Transport (CVT) method

In CVT technique, crystal is grown from vapour by means of chemical

reaction either in the gas phase or on the surface. This method is employed for

relatively non-volatile materials. The material to be crystallized is converted into one

or more gaseous product and moved towards the cold end either by diffusion or

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carried by a gas. The metal compounds are deposited on the surface of the substrate

either thermally or by chemical reduction. Production of thin layers of crystals

achieved by this method assumes commercial advantages.

1.5.3 Crystal growth by melt techniques

Melt growth is the process of crystallization by fusion and re-solidification of

the pure material. This is the fastest of the all crystal growth methods and is widely

used for the preparation of bigger and larger quantity of crystals [43-50]. Only those

materials, which melt congruently and have vapour pressure experimentally viable at

its melting point are grown by this method. The experimental arrangement is very

simple. Primarily the material to be grown is melted and afterwards progressively

cooled to yield crystalline form; this method has been utilized to produce

commercially important semiconductor metals and laser host crystals. The availability

of pure and perfect crystals is the main advantage of this technique. Let us consider

here the following techniques briefly.

 Bridgemann technique

 Czochralski technique

Bridgeman technique

Bridgeman technique has two versions viz. Horizontal Bridgeman method and

Vertical Bridgeman method. In Bridgeman technique, the withdrawing of boat or

capsule containing molten materials through a temperature gradient results in the

growth. This method is often utilized for growing the crystals of metals,

semiconductors and alkaline earth halides [51-53]. But materials having high melting

point and high expansion coefficient cannot be grown by this method.

Czochralski technique

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 This method is used in the case of materials having high melting and high

volume expansion coefficients associated with the solidification [54-56]. This

technique is an effective method for growing single crystal of semiconductors like

silicon and other materials.

Zone-growth method

Zone-growth method is employed for the growth of single crystals. Zone

melting is considered as refining or a purification technique. In this method, a part

called zone of the solid material is melted and this molted zone is allowed to pass

along the length of the charge together with the heating elements. When a crystal

freezes from the melt, it tends to reject impurities. If zone refining is applied to such

crystal, the resulting single crystal can be purer than the original. If more than one

pass is made, a high degree of purity of the crystal can be achieved. Such a process is

called zone refining. This process is suitable for the preparation of high purity silicon

and germanium [57].

1.5.4 Crystal growth by solution method

Growth of crystals from solution is the simplest and oldest of crystal growth

technique. In this method, the crystals are prepared from a solution at temperature

well below its melting point. This may help to grow crystals even at room temperature

and it will turn out to be more advantageous. Here the crystallization takes place from

the critically supersaturated solution. Supersaturation is achieved either by lowering

the temperature of the solution or by slow evaporation.

The main methods commonly used in this process are

(i) High temperature solution growth

(ii) Low temperature solution growth

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 A brief description of this method is discussed in the following section.

1.5.4 (a) High temperature solution growth

High temperature solution growth includes a number of related techniques

[58]. The flux method and liquid phase epitaxy are the two widely used methods. Flux

is a high temperature solvent and this reduces the melting temperature of the solute.

The main advantage of the flux growth is the reduction of high temperature. The

materials to be crystallized are dissolved in a proper solvent at a temperature slightly

above the saturation temperature and slow cooling of the container allows the growth

of crystals.

The following are the two methods under this category. They are

(i) Flux growth

(ii) Hydro-thermal growth

Hydrothermal growth

This is one of the best methods of growing certain class of materials, which

are practically insoluble in water up to its boiling point. It can be treated as aqueous

solution growth at elevated temperature and pressure. Autoclaves with gold or silver

linings are usually utilized for the growth process. The hot saturated solution is

directed towards the upper (colder) part where it cools and gets supersaturated

resulting in the growth of crystal. The solution simply acts as a transporting agent for

the solid phase. Synthetic quartz is grown by this technique [59].

Flux growth

In this technique, the solvent is a molten salt or oxide or a mixture [60]. The

preliminary need for crystal growth is the achievement of supersaturation and it is

created by temperature change. The solubility of most materials declines with

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reducing temperature so that cooling is often used to create supersaturation. In some

cases, the supersaturation is created by

i) Evaporation of solvent

ii) Changing the solvent composition

iii) Chemical reaction

The selection of the method to create supersaturation depends strongly on the

facilities available and the required quality, size, purity and homogeneity of the

crystals.

The flux growth is preferably used if

i) The material melts incongruently

ii) The melting point of the material is too high

iii) The material is non-stochiometric at its melting point due to high vapour
pressure of one or more constituents and
iv) A destructive phase transition is present closer to the melting point.

1.5.4 (b) Low temperature solution growth

Growth of crystals from aqueous solution is one of the ancient methods of

crystal growth. This method is discussed in detail giving all the required information

because the materials chosen for the present study have been grown from low

temperature solution. This method occupies a prominent place owing to its versatility

and simplicity. Materials which decompose on heating can be grown from the

solution growth if suitable solvents are available. This method is widely used to grow

bulk crystals [58]. After undergoing so many modifications and refinements, the

process of solution growth now yields good quality crystals for a variety of

applications. Growth of crystals from solution at room temperature has many

advantages over melt growth though the rate of crystallization is very low [61-64].

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 This method is well suited to those materials which suffer from decomposition in

the melt or in the solid at high temperature and which undergo structural

transformation while cooling from the melting point and as a matter of fact numerous

organic and inorganic materials which fall in this category can be crystallized using

this technique.

This low temperature solution growth technique also allows variety of different

morphologies and polymorphic forms of the same substance that can be grown by

variation of growth condition or of solvent. Since growth is carried out at room

temperature, the concentration of structural imperfections in solution growth crystal is

relatively low

Solvent selection

The impurities present in the solvent may be incorporated into the crystal lattice,

while growing the crystal, resulting in the formation of flaws and defects. Hence an

essential prerequisite for success in crystal growth is the availability of the solvent of

the highest purity attainable. Sometimes impurities may slow down the crystallization

process by being absorbed on the growing phase of the crystal, which changes the

crystal habit [65]. A careful repetitive use of standard purification methods of re-

crystallization followed by filtration of the solution would increase the level of purity.

In laboratory, the processes involved to purity the solvent are filtration, reverse

osmosis, deionization and ultra-filtration.

A solution is a homogeneous mixture of a solute in a solvent. Solute is the

component, which is present in a smaller quantity and that one which gets dissolved

in the solution. For a given solute, there may be different solvents. The solvents must

be chosen taking into account the following factors to grow crystals from solution.

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 a) Good solubility for the given solute

b) Good temperature co-efficient of solute solubility

c) Less viscosity and less volatility

d) Less corrosion and non-toxicity

e) Small vapour pressure

f) Cost advantage

Proper choice of solvent provides some control over crystal habit and this effect

depends on the interaction of the surface of the crystal as it grows and the solvent

molecules.

Solubility

Solubility of the material in a solvent decides the amount of material, which is

available for the growth and hence defines the limit of crystal size. Solubility

gradient is another important parameter, which dictates the growth procedure. A flat

or a steep solubility curve will not enable the growth of bulk crystals from solution. If

the solubility gradient is very small, slow evaporation of the solvent is the option for

crystal growth. Supersaturation is an important parameter for the solution of growth

process. The crystal grows by the accession of the solute in the solution as a degree

of supersaturation is maintained. The solubility data at various temperatures are

essential to determine the level of supersaturation. Hence, the solubility of the solute

in the chosen solvent must be determined before starting the growth process.

The solubility of the solute may be determined by dissolving the solute in the

solvent maintained at a constant temperature with continuous stirring. On reaching

saturation, the equilibrium concentration of the solute may be determined

gravimetrically. A sample of the clear supernatant liquid is withdrawn by means of a

16
warmed pipette and a weighed quantity of the sample is analyzed. The solubility

curve can be plotted after determining the concentration for different temperature.

Meir carried out the research on the relationship between supersaturation and

spontaneous crystallization [48] and the results can be represented as shown

diagrammatically in the figure 1.2.

Figure 1.2: Meir’s Solubility Diagram

The lower continuous line is the normal solubility curve for the salt concerned.

Temperature and concentration at which spontaneous crystallization occurs are

represented by the upper broken curve, generally referred to as the super solubility

curve. The diagram is divided into three zones, namely

a) The stable unsaturated zone where crystallization is not possible

b) The second region known as metastable zone, between the solubility and

super solubility curves where spontaneous crystallization cannot occur and

a seed crystal is essential to facilitate growth.

c) The third region known as the unstable or labile zone or supersaturation

region where spontaneous crystallization is more probable.

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Preparation of solution and seed

To prepare a saturated solution, it is necessary to have an accurate solubility –

temperature data of the material. Sintered glass filters of different pore sizes (50-100

μm) are used for solution filtration. The filtered solution is taken in a growth vessel.

The growth vessel is sealed to prevent solvent evaporation. The solution is tested for

saturation by suspending a small crystal in the solution. If the system is not in

equilibrium, the crystal will either dissolve or solute that will crystallize the seed. By

varying the temperature, the equilibrium is achieved. The test seed is then withdrawn

from the solution and a good quality seed is inserted. The temperature of the solution

is slightly raised above the saturated temperature to dissolve any unwanted nuclei or

any surface damage on the seed. The temperature is then lowered to the equilibrium

temperature and the growth commences.

Defects present in a seed propagate in to the bulk of its crystal, which decrease

the quality of its crystal. Hence, seed crystals are prepared with care. The quality of

the crystal is usually slightly better than that of the seed. Seed crystals are prepared

by slow evaporation of saturated solutions. During this process, the surface of the

seed inevitably gets damaged. This problem can be overcome by dissolving the

deformed surface layers of the seed crystal, before commencing the growth. Seeds of

good quality with low visible defects that are free from inclusions and imperfections

are selected for growth. The perfection of the final crystal is based on

the purity of the starting material, the quality of the seed crystal, cooling rate

employed and the efficiency of agitation.

Hence high quality crystals can be grown from quality seeds in an efficiently

stirred solution. So, the cooling rate and agitation are discussed below.

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Cooling rate and agitation

Temperature and super saturation have to be preciously controlled for desirable

results. The growth rate is maintained linear in order to grow large crystals. This

requires an increase in the supersaturating level but the linear cooling will not

provide this. Hence, after the initial growth, the rate of temperature lowering is

increased. Operation within the metastable limit occurs without any spurious

nucleation in the solution. A large cooling rate changes the solubility beyond the

metastable limit. Further, fluctuations in supersaturation may encourage solution

inclusion flaw in growing crystals. Hence a balance between the temperature

lowering rate and the growth has to be maintained.

To have a regular and even growth, the level of supersaturation has to be

maintained equally around the surface of the growing crystal. An uneven growth

leads to localized stresses at the surface generating imperfections in the bulk crystals.

Moreover, the concentration gradients that exist in the growth vessels at different

faces of the crystal cause fluctuations in supersaturation, and this will seriously affect

the growth rate of individual faces. The gradient at the bottom of the vessel exceeds

the metastable zone width, resulting in spurious nucleation. The degree of formation

of concentration gradients around the crystal depends on the efficiency of agitation of

the solution. This is achieved by agitating the saturated solution in either direction at

an optimized speed using a stirrer motor.

Crystal habit

The growth of a crystal at approximately equivalent rates along all the direction is

a prerequisite for its accurate characterization. This will result in a large bulk crystal

from which samples of any desired orientation can be cut. Further such large crystal

should also be devoid of dislocation and other defects. These imperfections become

19
isolated into defective regions surrounded by large volumes of high perfection, with

which the crystal grows as a bulk habit. In the crystals which grow as needles or

plates, the grown dislocations propagate along the principal growth direction and the

crystal remains imperfect [66].

Any one of the following ways can achieve changes of habits in such crystals

which naturally grow as needles or plates.

a) Changing the temperature of growth

b) Changing the pH of the solution

c) Adding a habit modifying agent

d) Changing the solvent.

Isothermal and non-isothermal methods

The isothermal methods include solvent evaporation or change in solvent

concentration, temperature differential and chemical or electrochemical reaction. A

non-isothermal method is slow cooling. Isothermal methods have an advantage over

non-isothermal is that the concentration of impurities (or dopants) in the grown crystal

is uniform.

Slow cooling method

In most of the laboratories, this method is used to grow bulk crystals. In slow

cooling method, supersaturation is produced by a change in temperature throughout

the whole crystallizer. The crystallizer is a thermostat or whose volume is selected

based on the desired size of the crystals and the temperature dependence of the

solubility of the substance since the volume of the crystallizer is finite and the amount

of substance placed in it is limited, that the supersaturation requires systematic

cooling. The crystallization process is carried out in such a way that the temperature

dependence of the concentration moves into the metastable region along the saturation

20
curve in the direction of lower solubility. Its main advantages are the need to use a

range of temperature. The possible range of temperature is usually small so that much

of the solute remains in the solution at the end of the run. To compensate this effect,

large volumes of solutions are required. The use of a range of temperature may not be

desirable because the properties of the grown material may vary with temperature.

Even though, the method has technical difficulty of requiring a programmable

temperature control, which is used with great success.

Slow evaporation method

In evaporation method the solution loses particles, which are weakly bound to

other components, and, therefore, the volume of the solution decreases. So, in this

method an excess of given solute is established by utilizing the difference between the

rates of evaporation of the solvent and the solute. The vapour pressure of the solvent

above the solution is higher than the vapour pressure of the solute, and therefore the

solvent evaporates more rapidly and the solution becomes supersaturated with non-

toxic solvents like water, and it is permissible to allow evaporation into atmosphere.

The evaporation techniques of crystal growth have an advantage that the crystals grow

at fixed temperature. This method is the only one, which can be used for materials,

which have a very small temperature co-efficient of solubility.

1.6 Ferroelectric and Nonlinear optical materials

As the samples of this work show both ferroelectric and nonlinear optical

(NLO) properties, some details of these materials are given here. Ferroelectric

materials have the properties such as spontaneous polarization on cooling below the

Curie point, ferroelectric domains and ferroelectric hysteresis loop. Ferroelectric

phenomenon was discovered in 1921 in the sample of Seignette or Rochelle salt. A

ferroelectric material has permanent electric dipoles. If mechanical stress is exerted at

21
the ends of the polar axis, the electrical charges of opposite sign accumulate at the two

ends of the polar directions. This leads to potential differences that are developing

between the two ends of the same specimen. This is known as piezoelectricity and it

was first discovered by Pierre Curie and Jacques Curie in 1881. A ferroelectric crystal

must be a piezoelectric but a piezoelectric crystal need not be a ferroelectric. A

change in temperature in a crystal having a polar axis produces positive and negative

charges at the opposite ends of the polar axis. This phenomenon is called

pyroelectricity. An external field can reverse the direction of positive and negative

charges from ends of pyroelectric crystals. This reversal is a function of strength of

the applied electric field, which leads to a ferroelectric phenomenon. The

piezoelectric and pyroelectric properties have a vital role in detecting the absence of

center of symmetry. Ferroelectric crystals have spontaneous polarization and show

piezoelectric effect and ferroelectric behaviour along a unique direction axis. The

spontaneous polarization is given by the value of the dipole moment per unit volume

or by the value of the charge per unit area on the surface perpendicular to the axis of

spontaneous polarization. The value of spontaneous polarization depends on the

temperature and it vanishes at Curie point. A ferroelectric is termed as displacive

when the elementary dipoles strictly vanish in the paraelectric phase.

All ferroelectric materials have a transition temperature called the Curie point

(Tc). At a temperature T< T c, the crystal exhibits ferroelectricity while above T >T c,

it is not ferroelectric. As the temperature decreases through the Curie point, the crystal

undergoes phase transition from non-ferroelectric phase to ferroelectric phase. Near

the Curie point or transition temperature, thermodynamic properties including

dielectric constant (εr) above the Curie point (T >Tc) in ferroelectric crystal is

governed by the Curie-Weiss law which is given by εr = C/(T-Tc) where εr is the

22
relative permittivity of the material, C is the Curie constant and Tc is the Curie

temperature [6, 67-69]. Ferroelectric crystals have attracted attention for applications

in many electric and electro-optic devices. Ferroelectric crystals possess regions with

uniform polarization called ferroelectric domains. Within a domain, all the electric

dipoles are aligned in the same direction and separated by interfaces called domain

walls. A ferroelectric single spontaneous polarization in ferroelectrics can be switched

by an applied electric field. At very high field levels, the polarization reaches a

saturation value. At zero external fields, some of the domains remain aligned in the

positive direction. Hence the crystal will show a permanent polarization. The external

field applied to depolarize completely is called the coercive field strength. If the field

is increased to a more negative value the direction of polarization flips and hence a

hysteresis loop is obtained.

Ferroelectric crystals utilize the unique dielectric, piezoelectric, pyroelectric and

electro-optic properties for device fabrication. Some of the most important electronic

applications of ferroelectric and pyroelectric crystals include non-volatile memories ,

thin film capacitors, pyroelectric sensors, Surface Acoustic Wave (SAW) substrates,

Ferroelectric Random Access Memories (FRAMs), transducers, switching devices

etc. The electro optic devices are optical wave guides, optical memories and displays.

The main advantages offered by FRAMs include non volatile and radiation hardened

compatibility with CMOS and GaAs circuitry, high speed (30 ns cycle time for read/

erase / rewrite) and high density. Data is stored by localized polarization switching in

the microscopic regions of ferroelectric thin films. The FRAMs are non-volatile

because the polarization remains in the same state even after the voltage is removed

[70-71].

23
 Franken and co-workers [72] generally identified the birth of nonlinear optics

with the experiment on Second Harmonic Generation (SHG) of light by ruby laser

pulse in a quartz crystal. An ultraviolet of wavelength = 347 nm was emitted from

the quartz crystal (frequency is doubled) for the incident wavelength (= 694 nm )

of ruby laser. After the invention of SHG process with the quartz crystal, a lot of

nonlinear optical (NLO) materials have been discovered and NLO materials play a

major role in nonlinear optics and in particular they have a great impact on

information technology and industrial applications. In the last decade, however, this

effort has also brought its fruits in applied aspects of nonlinear optics. This can be

essentially traced to the improvement of the performances of the NLO materials. The

understanding of the nonlinear polarization mechanisms and their relations to the

structural characteristics of the materials has been considerably improved. The new

development of techniques for the fabrication and growth of artificial materials has

dramatically contributed to this evolution [73].

NLO crystals can be classified into organic, inorganic and semi-organic

materials. Organic nonlinear optical materials have been investigated due to their

potentially high nonlinearities and rapid response in electro-optic effect compared to

inorganic NLO materials. These molecular organic compounds with one or more

aromatic systems in conjugated positions, leading to charge transfer systems have

been intensely studied for the past two decades. The conjugated π electron system

provides a pathway for the entire length of conjugation under the perturbation of an

external electric field. Fictionalization of both ends of the π conjugated system with

appropriate, electron donor and acceptor groups can increase the asymmetric

electronic distribution in either or both the ground and excited states, thus leading to

an increase of optical nonlinearity. When the acceptor and donor are placed at

24
terminal position of conjugated backbone, both linear and nonlinear optical properties

have increased significantly which involves correlated and high delocalized π electron

states. The strength of donor and acceptor groups and order of their stacking along

the backbone plays important roles in determining the magnitude of nonlinear optical

(NLO) efficiency. In case of organic crystals, two requirements are satisfied: i) they

are made of highly polarisable molecules, the so called conjugated molecules, where

highly delocalized -electrons can easily move between electron donor and electron

acceptor groups on opposite sides of the molecule, inducing a molecular charge

transfer and ii) the molecules are adequately packed to build up a non-centro

symmetrical crystal structure that provides non-vanishing second order nonlinear

coefficients. The organic NLO materials play an important role in SHG, frequency

mixing, electro-optic modulation, optical parametric oscillation, optical bi-stability

etc. Organic crystals have parameters superior to widely used crystals like KDP. The

organic NLO materials have an order of magnitude having higher efficiency, second

harmonic generation, and also exhibit substantially greater laser damage thresholds

[74].

In the beginning of 1961, studies were concentrated on inorganic materials such

as quartz, potassium dihydrogen phosphate (KDP), potassium titanyl phosphate

(KTP) and its analogues and semiconductors such as cadmium sulfide, selenium, and

tellurium. Many of these materials have been successfully used in commercial

frequency doublers, mixers and parametric generators to provide coherent laser

radiation at high efficiency in new regions of the spectrum inaccessible by other

nonlinear optical crystal and conventional laser sources. Inorganic crystals are ionic

bonded, it is always easier to synthesize inorganic materials. However, inorganic

crystals face a trade-off problem between response time and magnitude of optical

25
nonlinearity. High temperature oxide materials are studied for device application like

piezoelectric, ferroelectric and electro-optics. Inorganic compounds have property

such as they are ionic bonded molecules and soluble in water. They are sensitive to

pH and ionic strength and are highly thermally stable. They have an excellent

property in X-ray diffraction quality and also extremely good in their mechanical

strength. Inorganic and organic materials are combined to form semi organic crystals.

They share the properties of both organic and inorganic materials. Recent interest is

concentrated on metal complexes of organic compounds owing to their large

nonlinearity. The approach of combining the high nonlinear optical coefficients of the

organic molecules with the excellent physical properties of the inorganic has been

found to be overwhelmingly successful in the recent past. Hence, a search is

concentrated on semi-organic materials due to their large nonlinearity, high resistance

to laser induced damage, low angular sensitivity and good mechanical hardness

[75,76].

1.7 Review of literature

Crystals of amino acids and their complexes can be considered for a variety of

applications. Among amino acids, glycine (NH2CH2COOH) is the simplest amino

acid and has normally three polymeric crystalline forms viz . α-glycine, β-glycine

and γ-glycine [77-83]. The least stable form β-glycine is related to the other two

polymorphs. It is always obtained from a water-alcohol mixed solvent and can

transform rapidly to the α-form in the presence of water or upon heating. The α-form

is the metastable form at ambient temperature, which spontaneously crystallizes from

water, or may be obtained by evaporation of aqueous solutions. Some of the glycine

complexes in single crystalline forms have been grown and studied by many

researchers [84-92]. When glycine combines with sulphuric acid in the molar ratio

26
3:1, Triglycine Sulphate (TGS) crystal is formed and it is a suitable material for

developing detectors of infrared radiation and target faces in vidicons based on the

pyroelectric effect. Doping crystals with various kinds of dopants influences the

solubility, growth rate, morphology, structural, electrical and other properties of the

crystals [93-101]. Studies on various physical and chemical properties of undoped and

divalent impurity doped TGS crystals have been reported in the literature [102-115].

But there are no research papers found in the literature dealing with the properties of

TGS crystals admixtured with some organic and inorganic acids such as nitric acid,

picric acid, percholoric acid and salicylic acid with different concentrations (10 mol

%, 20 mol %, 30 mol %). Hence a research programme has been carried out to study

the various physical and chemical properties of pure and some organic and inorganic

acids admixtured TGS crystals.

Triglycine sulphate (TGS) crystals are used in the fabrication of capacitors, IR

detectors, pyroelectric detectors, transducers, sensors etc . The crystal system for TGS

is monoclinic below and above the Curie temperature (49.5▫C). The space group

transforms from P21 in the ferroelectric phase to centrosymmetrical P2 1/m in the

paraelectric phase [116-120]. The cleavage plane is generally expressed as the b-plane

and the spontaneous polarization arises along the crystallographic b-axis. However,

the TGS crystal has a disadvantage that it gets depolarized by thermal, electrical or

mechanical means. TGS single crystals can easily be grown from aqueous solutions

and exhibit, among ferroelectric materials is one of the best examples of second order

phase transition with order-disorder character [121-124]. Despite the complex crystal

and chemical structure, the ferroelectric phase transition in TGS follows perfectly

field behaviour and the static dielectric properties can be quantitatively described by a

simple Landau-Devonshire model. The material is therefore ideal to test a wide range

27
of theoretical predictions on critical phenomena and has been extensively studied in

the past [125-132]. It has been observed by many researchers that the undoped TGS

crystals have some disadvantage over doped ones such as: (i) the ferroelectric

domains that possess high mobility at room temperature, (ii) easy depolarization by

electrical , mechanical and thermal means , (iii) microbial contamination with time

during the growth , (iv) low curie point , etc [133-136].

In order to overcome above disadvantages, variety of dopants such as amino

acids, organic and inorganic compounds have been introduced in TGS crystal to

achieve effective internal bias to stabilize the domains and desired pyroelectric and

ferroelectric properties [137-143]. Many metallic ion dopants such as Fe 3+, Cr3+, Mn2+,

Ni2+, etc have added to modify the properties of TGS. Rare earth metal ions such as

La , Ce and Nd modified the morphology and coercive field values [144-149]. TGS is

an example of hydrogen bonded crystals also SO4 group in TGS may be considered

equivalent to the PO4 group in KDP (Potassium dihydrogen orthophosphate) and ADP

(Ammonium dihydrogen orthophosphate) single crystals [150-153]. The effects

LiSO4, acetic acid, n-bromo-succinimide and other dopants on the growth and

characterization of Triglycine sulphate (TGS) crystals have been reported in the

literature [154-172].

1.8 Objectives

In this research work, growth of pure TGS crystals and TGS crystals admixtured

with nitric acid, picric acid, percholoric acid and salicylic acid separately with

different concentrations was carried out by solution method and various properties of

the grown crystals were studied. The important objectives of this work are given

below.

28
 (a) To synthesize and grow pure TGS crystals and TGS crystals admixtured

with some organic and inorganic acids by solution method

(b) To carry out solubility and nucleation kinetics studies

(c) To carry out single crystal and powder XRD studies for the grown
crystals.
(d) To perform spectroscopic studies such as FTIR and UV-visible

transmittance on the grown crystals

(e) To carry out dielectric studies on the grown crystals

(f) To measure AC conductivity and activation energy of the grown crystals.

(g) To measure density of the grown crystals

(h) To carry out NLO properties of the grown crystals

(i) To determine the hardness number, work hardening coefficient, Stiffness

constant and yield strength of the grown crystals

(j) To perform the Energy Dispersive Analysis by X-rays (EDAX) to

determine the chemical composition of the grown crystals

(k) To carry out Thermogravimetric and Differential Thermal Analysis

(TG/DTA) to investigate the thermal stability of the grown crystals.

(l) To carry out ultrasonic studies for the saturated solutions of the samples of

this work.

1.9 Scope of the thesis

Synthesis of pure and some organic and inorganic acids admixtured Tri Glycine

Sulphate (TGS) salts were carried out. In order to obtain the desired compounds

aqueous solutions of suitable molecular proportions of chemical components were

reacted under continuous stirring in a hot plate magnetic stirrer. Nucleation Kinetic

studies of the synthesized salts were performed for the selected supersaturation ratios.

29
The fundamental and experimental aspects of solution growth technique have been

explained. The single crystals of the synthesized pure and some organic and inorganic

acids admixtured TGS salts were grown from aqueous solutions by slow evaporation

method. The grown crystals were characterized structurally and physically by

carrying out X-ray diffraction, thermal, mechanical, electrical and spectroscopic

studies.

Unit cell parameters were obtained by single crystal X-ray diffraction (XRD)

and powder XRD methods. Thermal analyses were carried to check the thermal

stability of the grown crystals. Fourier Transform infrared (FTIR) spectral studies

have been carried out to identify the presence of functional groups of the samples.

Microhardness measurements have been carried out and the Vickers microhardness

number and work hardening coefficient were determined. The dielectric constant

(εr), dielectric loss factor (tan δ), AC electrical conductivity (σac) and activation

energy were determined to understand the dielectric nature of the grown crystals.

UV-visible spectra studies explain the transmission ability of the grown crystals.

EDAX analysis confirms the incorporation of dopants into the lattice of pure crystal.

The Nonlinear Optical (NLO) property of the grown crystal was confirmed by

Kurtz-Perry powder technique and a study of its second harmonic generation

efficiency in comparison with potassium dihydrogen phosphate (KDP) crystal has

been made.

A report of the present investigations is provided in this thesis. The first chapter

deals with general introduction, a brief review of various studies made on TGS single

crystals in the recent past, objectives of the present investigation and scope of the

present work.

30
 The second chapter explains the experimental details about the different

growth methods and the instrumentation for various characterization techniques.

Growth and studies on various properties of pure TGS and nitric acid

admixtured TGS single crystals are covered in the third chapter.

The fourth chapter provides growth and characterization of TGS single

crystals admixtured with picric acid. The fifth chapter gives the growth and studies of

percholoric acid admixtured TGS single crystals.

Growth and characterization of salicylic acid admixtured TGS single crystals

are reported in the sixth chapter and the summary of conclusions derived out of the

present study along with the scope for future work in the same area of research are

presented in the seventh chapter. Finally, the references cited are listed. The resume of

the candidate, list of publications and list of seminars/conferences attended/presented

31
 CHAPTER-II
EXPERIMENTAL TECHNIQUES
2.1 Introduction

This chapter deals with theory part and experimental part in connection with

the solubility and nucleation kinetics. The details of various instruments adopted to

carry out different studies of the grown samples are also given in this chapter.

2.2 Solubility by gravimetrical method

The amount of solute dissolved in 100 g of a solvent to form a saturated solution

at a given temperature is termed as the solubility of the substance in the solvent at that

temperature. The solubility of the grown materials is studied by gravimetric analysis.

The solubility of the solute can be determined by dissolving the solute in the solvent

maintained at a constant temperature with continuous stirring. On reaching saturation,

the equilibrium concentration of the solute can be determined gravimetrically. A

sample is added step by step to 50 ml of double-distilled water in an air-tight

container kept on the hot-plate magnetic stirrer and stirring is continued till a small

precipitate is formed. This gives confirmation of supersaturated condition of the

solution. Then, 10 ml of the solution is pipetted out and taken in a petri dish and it is

warmed up till the solvent gets evaporated out. By measuring the amount of solute

present in the petri dish, the solubility (in g/100 ml) of the sample is determined. The

solubility of a solute in a given solvent varies appreciably with temperature. The

curves drawn between the solubility and temperature are called solubility curves.

2.3 Nucleation kinetics

2.3.1 Classical theory

In general, crystal growth process takes place by three steps a) the achievement of

supersaturation or super cooling; b) the formation of crystal nuclei and c) the growth

32
of crystal. To prepare supersaturated solution, the first and foremost thing is to study

the solubility of the substances. Solubility data are necessary to carry out nucleation

kinetic studies and the knowledge of nucleation will be very much useful to control

the growth kinetics for obtaining good quality crystals.

Nucleation is an important phenomenon in crystal growth and it is the precursor

of the crystal growth. Nucleation may occur spontaneously or it may be induced

artificially. Generally two types of nucleation namely homogeneous and

heterogeneous nucleation are considered in the crystal growth. Theories of nucleation

were developed by many authors such as Becker and Doring, Turnbull and Fishner

and Gibbs and others [173-177]. Most of the modern nucleation theories are based on

Gibbs ideas which are known as classical theory of nucleation. When a few atoms,

ions or molecules join together in a supersaturated solution, a cluster or nucleus is

formed and the overall excess free energy (G) between the nucleus and solute in the

supersaturated solution is given by

DG= DGs + DGv (1)

where DGs is the surface excess free energy and DGv is the volume excess free energy.

Once the nucleation occurs in the supersaturated solution, the nucleus grows quickly

and a bright sparkling particle is seen. The time of observation of the sparkling particle

in the undisturbed solution from the time at which the solution reaches experimental

temperature is called the induction period (t). For a given volume of solution, the

frequency of formation of nuclei is inversely proportional to the induction period. The

expression for the induction period in terms of Gibbs’ free energy is given by

ln t = −B+ ∆ G (2)
kT

33
 where B is a constant, k is the Boltzmann’s constant, and T is the absolute

temperature. Usually nucleus formed in supersaturated solution is assumed to be

spherical in shape. According to Gibbs’ theory, in terms of surface thermodynamics,

the overall excess free energy between a spherical nucleus and solute can be written

as

DG = 4pr2 s + (4/3)pr3 DGv (3)

Where DGv is the free energy change per unit volume, r is the radius of the

nucleus and s is the interfacial tension or surface energy per unit area. This energy

will be maximum for certain value of r, which is known as critical radius. Nuclei

formed with radius greater than this r are stable and decrease their free energy by

growing. According to Thomson-Gibbs equation, the volume excess free energy is

given by

DGv = (kT/v) ln S (4)

where S is the supersaturation ratio and v is volume of a molecule. S is given by

S = C/Co where C is the supersaturated concentration and Co is the saturated

concentration. The net free energy change (DG) increases with the increase in size

of nucleus, attains maximum and decreases with further increase in the size of

nucleus. The size corresponding to the maximum free energy change is called

critical nucleus [58]. The radius of the critical nucleus can be obtained by setting

the condition

d (DG) / dr = 0

By differentiating equation (3) we get,

d (DG) / dr = 8pr s + 4pr2 DGv’ = 0

34
The size of critical nucleus is obtained by putting r = r1 ,

r1 = -2 s / DGv’ (5)

Substituting equation. (4) in the equation (5), we get

r1 = -2 sv / kT ln S

Since Nk = R and taking modulus

r1 = 2 sv N / RT ln S (6)

where R is the universal gas constant and N is the Avogadro’s number.

Substituting equation (6) in the equation (3), we get Gibbs’ free energy change for

the critical nucleus (Here r = r1 and DG =DG1 for critical nucleus)

DG1 = (16 p s3 v2 N2) / [3R2 T2 (ln S)2 ] (7)

Therefore, equation (2) can be written for critical nucleus as

ln t =-B + (16 p s3 v2 N3) / [3R3 T3 (ln S)2 ] (8)

A plot of 1/ (ln S)2 against ln t from equation (8) is straight line and the

slope is

m = (16 p s3 v2 N3) / (3R3 T3 ) (9)

Therefore, DG1 = mRT / [N (ln S)2 ]

or DG1 = mkT / (ln S)2 (10)

From equation (9) we have,

s = (RT/ N) [3m / (16 pv2)]1/3 (11)

The number of molecules in a critical nucleus is found using the following

equation

35
 n = (4/3) (p/v) r13 (12)

Thus, the equations for critical nucleation parameters could be determined using the

above formulae.

2.3.2 Measurement of induction period

The critical nucleation parameters for samples have been calculated using the

values of induction period [178]. A constant temperature bath (controlled to an

accuracy of + 0.01o C) and a corning glass breaker were used for this study. The

corning glass breaker is used as a nucleation cell. The nucleation cell (100 ml corning

glass beaker) is fixed in the constant temperature bath and the system is illuminated

using a powerful lamp for observing the nucleation. Supersaturated aqueous solution

of the sample is taken in the nucleation cell and the constant temperature bath is

adjusted to a temperature slightly higher than the experimental temperature. As the

nucleation cell reaches the experimental temperature (30o C), time is noted. Once the

nucleation occurs in the cell, it grows quickly and a bright sparkling particle is seen

and the time is again noted. The time difference is known as the induction period and

it is measured by isothermal method at different supersaturation ratios [179].

2.4 Characterization techniques

In order to study the properties of the grown crystals, it is necessary to

involve the crystals for various characterizations. The usage of crystals depends

on the properties and so the characterization is an important part in crystal growth.

The instrumentation details and operating procedure of important characterization

techniques used in the present work are given in the following sections.

36
2.4.1Experimental method for crystal structure analysis
X-ray diffraction (XRD) method is used for analyzing the crystal structure of

the grown crystals and it is an efficient analytical technique used to identify and

characterize the crystalline materials. X-ray diffraction technique is the most

definitive one available for the determination of crystal and molecular structures and

the results obtained are usually unambiguous and generally quite accurate. Both

Single crystal XRD and Powder XRD methods are used to identify the crystal

structures. Powders of crystalline materials diffract X-rays. The powdered sample of

the grown crystal is ground using an agate mortar and it is spread in a sample holder.

It is allowed to rotate with respect to the impinging X-ray beam. The diffracted X-ray

photons are recorded by a scintillation counter, which is connected to an electronic

counting system and it is synchronized with a strip chart recorder, a rate meter, a

timer, a goniometer and a pulse height analyzer. These are often connected to a digital

printer to get the printed diffraction pattern. Powder X-ray diffraction provides less

information than single crystal X-ray diffraction; however, it is much simpler and

faster. Powder X-ray diffraction is useful for confirming the identity of a solid

material and determining crystallinity and phase purity. Both the methods of XRD

can be carried out for the grown crystals to check the correctness of the results

of the crystalline data [180,181]. In powder XRD technique, a monochromatic X-ray

beam be incident at Bragg angle θ on the set of lattice planes with interplanar

spacing(d) in a particular crystallite, so that the Bragg’s diffraction condition (2d sin

θ = nλ) for the lattice planes is satisfied.

The experimental arrangement of powder XRD method is shown

in Figure 2.1. The source of X-rays is made monochromatic by a filter. This allows

the X-ray beam to fall on the powdered specimen P through the slits S1 and S2. The

37
function of these slits is to get a narrow pencil of X-rays. Fine powder P, mixed with

gum is suspended vertically in the axis of a cylindrical camera. This enables sharp

lines to be obtained on the photographic film in the form of a circular arc. The X-

rays after falling on the powder; passes out of the camera through a cut in the film so

as to minimize the flogging produced by the scattering of the direct beam.

Figure 2.1: The experimental arrangement of powder X-ray

diffractometer

The single crystal X-ray diffractometer (typical diffractometer is shown

in Figure. 2. 2) is used for measuring X-ray diffraction data like unit cell

parameters, space groups and molecular structure of crystalline solids. The facility

can also be utilized for Miller indexing of different faces of crystals. Space group

tells us the symmetry with which molecules are arranged within the unit

cell. Coordinates of individual atoms of the molecules which can be

obtained from diffraction studies constitutes the structure. All the geometrical

features of molecules (bond distances, bond angles, torsion angles about bonds,

dihedral angle between planes etc.) may be obtained from coordinates [182,183].

38
 In most cases, the properties of a single crystal are not only a function of the

type and the quality of the crystals, but strongly dependent on its orientation. The

determination of single crystal orientation thus represents an essential step towards

the successful use of single crystals in technological applications. In the present

work, single crystal XRD data for the grown crystals were collected using

Bruker-Nonius MACH3/CAD4 diffractometer with MoK radiation (λ = 0.71073

Å) at School of Physics, Madurai Kamaraj University, Madurai. Powder XRD

patterns of the grown crystals were recorded at NIIST, Tiruvananthapuram using

a powder X-ray diffractometer (Copper target, Nickel filter, 1.54056 Å, 35 kV, 10

mA, Model: PANalytical XPERT – PRO).

39
2.4.2 Fourier Transform Infrared (FTIR) spectral technique
Infrared spectroscopy is the study of the interaction of infrared light with matter.

FTIR technique is most useful for identifying functional groups of organic and

inorganic compounds. It can be applied for the analysis of solids, liquids and gases.

The term Fourier Transform Infrared (FTIR) spectroscopy refers to fairly recent

development in which the data is collected and converted from an interference

pattern to a spectrum. Today’s FTIR instruments are computerized which makes

them faster and more sensitive than the older dispersive instruments. FTIR

spectroscopic technique is based on the principle of Michelson Interferometer with a

sensitive infrared detector and a digital minicomputer. FTIR spectrometers provide

higher resolution, total wavelength coverage, higher accuracy in frequency and

intensity measurements. The instruments also possess greater ease and speed of

operation. By interpreting the infrared absorption spectrum, the functional

groups of a compound and chemical bonds in a molecule can be determined.

FTIR spectra of pure compounds are generally so unique that they are like a

fingerprint. While organic compounds have detailed spectra, inorganic compounds

are usually much simpler.

Solid samples can be milled with potassium bromide (KBr) to form a very fine

powder. This powder is then compressed into a thin pellet which can be analyzed

[184-189]. In the present work, the FTIR spectra of the samples were recorded in

KBr matrix using Perkin Elmer Fourier Transform Infrared spectrometer (Model:

Spectrum RXI shown in Figure 2.3) at St. Joseph’s College (Autonomous), Trichy.

40
 Figure 2.3: Perkin Elmer Fourier Transform Infrared spectrometer

2.4.3 UV-visible spectral technique

The wavelength of visible light ranges from 400 nm-700 nm. The visible

region however, is a very small part of the entire electromagnetic spectrum.

Wavelengths slightly shorter than those of the visible region fall into the ultraviolet

region. A UV-visible spectrophotometer, for example, allows light of a given

frequency to pass through a sample and detects the amount of transmitted light. The

instrument compares the intensity of the transmitted light with that of the incident

light. The source of radiation in UV-visible spectrophotometer is a tungsten,

hydrogen or deuterium lamp. A source of radiation must be provided with each

spectral region having its own requirements. All spectrophotometers include some

way discriminable between different radiation frequencies either through the

use of filters, prisms, or through gratings. The polychromatic radiation is

separated into its component wavelength using monochromators which consists of

a prism and a plane grating. The sample absorbs a portion of the incident radiation

and the remainder is transmitted on to a detector where it is changed into an

electrical signal and displayed, usually after amplification, on a meter, chart

recorder, or some type of readout device.

41
 Automatic instruments gradually and continuously change the frequency or

wavelength. The spectrum of a compound represents a group of either wavelength or

frequency, continuously changing over a small portion of the electromagnetic

spectrum versus either the percentage of transmission (%T ) or the percentage of

absorbance (%A) [190,191]. UV-Visible transmittance spectra of the grown

crystals were recorded using a Perkin Elmer Lamda 35 spectrometer (Figure

2.4) in the range 190 nm - 1100 nm covering the near UV, visible, near infrared

region to find the transmission range for optical applications.

42
 Figure 2.4: UV-visible spectrophotometer

2.3.4 Density by floatation method

The floatation method was employed for the precise determination of density

and this method is sensitive to point defects and insensitive to dislocations of

crystals. Bromoform (density: 2.89 g/cc) and xylene (density: 0.99 g/cc) were used

for the experiment. After mixing the liquids- bromoform and xylene, in suitable

proportion in a specific gravity bottle, small piece of crystal was immersed in the

mixture of liquids. When the sample attains a state of mechanical equilibrium, the

density of the crystal would be equal to the density of mixture of liquids. The density

was calculated using the relation ρ= (W3-W1)/ (W2-W1) where W1 is the weight

of the empty specific gravity bottle, W2 is the weight of the specific gravity bottle

with full of water and W3 is the weight of specific gravity bottle with full of mixture

of bromoform and xylene. The density is also confirmed from the crystallographic

data using the relation ρ = (M.Z) / (N.V) where M is the molecular weight of the

material used, Z is the number of molecules per unit cell, N is Avogadro’s number

and V is the volume of the unit cell [193-196].

2.4.5 Kurtz-Perry powder technique

Kurtz-Perry technique was used to test the NLO activity of the samples. A

quantitative measurement of the conversion efficiency of the sample can be

determined by the modified version of powder technique developed by Kurtz

and Perry [197]. The experimental set-up is shown in Figure 2.5. The crystal is

ground into powder and it is packed densely between two transparent glass slides.

43
Nd:YAG laser is used as the light source and the fundamental laser beam of 1064

nm wavelength, 8 ns pulse in depth with 10 Hz pulse rate is made to fall normally on

the sample cell. The power of the incident beam is measured using a power meter

and it is 0.68 J/pulse. The transmitted fundamental wave is passed over a

monochromator (Czerny turney monochromator), which separates 532 nm (SHG

signal) from 1064 nm and are absorbed by CuSO4 solution. The filter F1

removes the 1064 nm light. F2 is a BG-38 filter, which also removes the residual

1064 nm light. F3 is an interference filter with bandwidth of 4 nm and central

wavelength 532 nm. The green light is detected by a photo multiplier tube (Hamatsu

RC 109, a visible PMT) and displayed on a storage oscilloscope . KDP crystal is

powdered to identical size and is used as the reference material for the SHG

measurement.

Figure 2.5: Experimental arrangement for Kurtz powder

method

2.4.6 Experimental for microhardness studies

Hardness of a material is the resistance offered to indentation by a much

harder body. It may be termed as a measure of the resistance against lattice

destruction or the resistance offered to permanent deformation or damage

44
[198]. It is the resistance to penetration to a metallurgist, the resistance to wear to

a lubrication engineer, the resistance to scratching to a mineralogist and the

resistance for cutting to a machinist. All these are related to the plastic flow stress

of the material. The hardness properties are basically related to the crystal

structure of the material. Microhardness study of the crystals brings out an

understanding of the plasticity of the crystal [199].

Hardness is a technique, in which a crystal is subjected to a relatively high

pressure within a localized area. By suitable choice of indenter material and

relatively simple equipment construction, hardness tests can be easily carried out

on all crystalline materials under various conditions of temperature and pressure.

2.4.6.(a) Methods of Hardness Test

Hardness measurement can be carried out by various methods. They are

classified as follows

• Static indentation test

• Dynamic indentation test

• Scratch test

• Rebound test

• Abrasion test

The most popular and simplest form is the static indentation test wherein the

specific geometry is pressed into the surface of a test specimen under known load.

The indenter may be a ball or a diamond cone or a diamond pyramid. Upon the

removal of the indenter, a permanent impression is retained in the specimen. The

hardness is calculated from the area or depth of indentation produced. The

variables are the type of indenter or load.

45
 In this static indentation test the indenter is pressed perpendicularly on the

surface of the sample by means of an applied load. Then by measuring the cross

sectional area or the depth of the indentation and knowing the applied load

empirical hardness number may be calculated. This procedure is followed by

Brinell, Meyer, Knoop and Rockwell test [ 200-202]. In the dynamic indentation

test, a ball or a cone (or a number of small spheres) is allowed to fall from a

definite height and the hardness number is obtained from the dimensions of the

indentation and the energy of impact.

The scratch test can be classified into two types:

• Comparison test is one, in which one material is said to be

harder than another if the second material is scratched by the first.

• A scratch test is done with a diamond indenter on the surface at a

steady rate and under a definite load. The hardness number is

expressed in terms of the width of depth of the groove formed.

In the rebound test an object of standard mass and dimensions is

bounced from the test surface and the height of rebound is taken as the measure of

hardness. In abrasion test, a specimen is loaded against a rotating disk and the rate

of wear is taken as the measure of hardness.

2.4.6. (b)Vickers hardness test

Among the various methods of hardness measurements discussed above, the

most common and reliable method is the Vickers hardness test method. In this

method, micro indentation is made on the surface of a specimen with the help of a

diamond indenter (Figure 2.6).

46
 Figure 2.6: Schematic diagram of Vickers diamond pyramid
indenter and indentation produced

Vickers pyramid indenter with the opposite faces contain an angle ( α = 136o )

is most widely accepted pyramid indenter. A pyramid is suited for hardness tests

due to the following two reasons [203].

• The contact pressure for a pyramid indenter is independent of the

Indent size.
• Pyramid indenters are less affected by elastic release than other

indenters. The base of the Vickers pyramid is a square and the depth of

indentation corresponds to 1/7 t h of the indentation diagonal. Hardness is

generally defined as the ratio of the load applied to the surface area of the

indentation.

The Vickers hardness number (Hv ) or Diamond Pyramidal Number (DPN) is

defined as

where α is the apex angle of the indenter (α = 136°).

47
 The Vickers hardness number is therefore calculated from the relation

Hv = 1.8554 P/d2 kg/ mm2

Where P is the applied load in kg and d is the diagonal length of the

indentation mark in mm [204-205]. The relationship between load (P) and diagonal

length (d) of indentation is given by P = a d n. This is called Meyer’s law [206]. Here

a and n are constants for a particular material, P and d are respectively the applied

load and average diagonal length of the indentation impression. Plots of log (P)

versus log (d) are drawn and from the slopes of these plots, the work hardening

coefficient (n) is determined. In the present study, the microhardness measurements

were made using Shimadzu microhardness Tester (Figure 2.7).

Figure 2.7: Shimadzu microhardness tester

2.4.7 Energy Dispersive Analysis by X-rays (EDAX) technique

Energy Dispersive Analysis by X-rays (EDAX) is a chemical microanalysis

48
technique used in conjunction with scanning electron microscopy (SEM). This

technique detects X-rays emitted from the sample during bombardment by an

electron beam to characterize the elemental composition of the analyzed

volume. The data generated by EDAX analysis consist of spectra showing

peaks; corresponding to the elements making up the true composition of the sample

being analyzed. When the sample is bombarded by the SEM's electron beam,

electrons are ejected from the atoms comprising the sample’s surface. The resulting

electron vacancies are filled by electrons from a higher state, and an X-ray is emitted

to balance the energy difference between the two electrons states. The X-ray energy

is characteristic of the element from which it was emitted. The EDAX detector

measures the relative abundance of emitted X-rays versus their energy. The spectrum

of X-ray energy versus counts is evaluated to determine the elemental composition

of the sample. The sample X-ray energy values from the EDAX spectrum are

compared with known characteristic X-ray energy values to determine the presence

of an element in the sample. Elements with atomic numbers ranging from that of

beryllium to uranium can be detected. The minimum detection limits vary from

approximately 0.1 to a few atom percent, depending on the element and the sample

matrix.

In the present study, EDAX studies were performed using the EDAX

detector (model-Thermo electron Corporation with super dry/II) equipped in

Hitachi model S-3000H scanning electron microscope (Figure 2.8).

49
 Figure 2.8: Scanning Electron Microscope - HITACHI Model S-
3000H
2.4.8 Experimental method for TG/DTA analysis
Thermogravimetric and Differential Thermal analysis (TG/DTA) are the

important thermal analyzing methods for characterizing the crystals. Thermal

analysis is a branch of materials science where the properties of materials are

studied as they change with temperature. TG/DTA technique provides a quantitative

measurement of any weight changes associated with thermally induced transitions.

For example, TG can record directly the loss in weight as a function of temperature

or time (when operating under isothermal conditions) for transitions that involve

dehydration or decomposition. Thermo Gravimetric curves are characteristic of a

given compound or material due to the unique sequence of physical transitions and

chemical reactions that occur over definite temperature ranges. TG data are useful in

characterizing materials as well as in investigating the thermodynamics and kinetics

of the reaction and transitions that result from the application of heat to these

materials. The usual temperature range for TG study is from ambient to 1100 oC in

either inert or reactive atmospheres. In TG, the weight of the sample is continuously

recorded as the temperature that is increased. Samples are placed in a crucible or

50
shallow dish that is positioned in a furnace on a quartz beam attached to an

automatic recording balance. Linear heating rates from 5 oC/min. to10 oC/min. are

typical. Computer software allows the computation of weight change which is

important in kinetic interpretations of reactions and processes.

In Differential Thermal Analysis (DTA), the difference in temperature

between the sample and a thermally inert reference material is measured as a

function of temperature (usually the sample temperature). Any transition that

the sample undergoes results in liberation or absorption of energy by the

sample with a corresponding deviation of its temperature from that of the

reference. A plot of the differential temperature, ∆T, versus the programmed

temperature, T, indicates the transition temperature whether the transition is

exothermic or endothermic. DTA and TG analyses are often run simultaneously

on a single sample [207,208].

In the present work TG and DTA studies on the grown crystals have

been carried out using SDT Q600 V 8.3 (Universal V4.7A TA) thermal

analyzer ( Figure 2.9 ) in the temperature range 30 oC – 1010 oC.

51
 Figure 2.9: Thermal analyzer Model: SDT Q600 V 8.3

2.4.9 Instrumentation for dielectric measurement

Atomic polarization , orientational polarization, space charge polarization

and electronic polarization can be easily understood by studying the dielectric

properties as a function of frequency and temperature. The frequency

dependence of these properties gives a great insight into the material applications.

The capacitance (Cs) and dielectric loss factor (tan δ) measurements were carried out

to an accuracy of ±2% using LCR meter (Agilent 4284A) (shown in Figure 2.10)

with three different frequencies at various temperatures [209-211]. The temperature

was controlled to an accuracy of ±1 °C. Air capacitance (Ca) was also measured.

Figure 2.10: Experimental arrangement for dielectric studies

The dielectric constant of the crystal was calculated using the relation

ε r = C s / Ca

The AC conductivity (σac) was calculated using the relation

σ ac = ω εr εo tan δ
where εo is the permittivity of free space (8.85 x 10-12 C2N-1m-2) and ω is the

52
angular frequency.

Activation energy

The general relation proposed by Arrhenius for the temperature variation of

conductivity of insulators is given by

σac = σo exp (-Eac/kT)

Where σo is a constant depending on the material, Eac is the activation energy, T is

the absolute temperature and k is the Boltzmann’s constant. The above equation may

be rewritten as

ln σ ac = ln σo (-E ac/kT)

A plot of ln(σ ac) versus 1/T gives [-E ac / K] as the slope and ln(σ o) as the

intercept. Values of ln(σac) were plotted against 1000/T for the grown samples and

the activation energy values were calculated from the slope of the straight line best

fitted by least square analysis.

2.4.11 Ultrasonic studies

An ultrasonic interferometer is a simple and direct device to determine the

velocity of ultrasonic waves in liquid with a high degree of accuracy. Here the high

frequency generator generates variable frequency, which excites the quartz placed at

the bottom of the measuring cell. The excited quartz crystal generators ultrasonic

waves in the experimental liquid. The liquid will now serve as an acoustical grating

element. Hence when ultrasonic waves passes through the rulings of grating

successive maxima and minima occurs, satisfying the condition for diffraction.

The measuring cell is connected to the output terminal of the high frequency

generator through a shielded cable. The cell is filled with the saturated solution

before switching on the generator. Now, the ultrasonic waves move normal from the

quartz crystal till they are reflected back by the movable reflector plate. Hence,

53
standing waves are formed in the solution in between the reflector plate and the

quartz crystal (Figure 2.11).

The distance between the reflector and crystal is varied using the micrometer

screw such that the anode current of the generator increases to a maximum and then

decreases to a minimum and again increases to a maximum. The distance of

separation between successive maximum or successive minimum in the anode

current is equal to half the wavelength of the ultrasonic waves in the solution.

Therefore by noting the initial and final position of the micrometer screw for one

complete oscillation, the distance moved by the reflector can be determined.

Figure 2.11: Ultrasonic Interferometer (Model: PICO F41)

To minimize the error, the distance (d) moved by the micrometer screw is noted

for ’x’ number of oscillations by noting the initial and final reading in the

micrometer screw. From the total distance (d) moved by the micrometer screw, the

number of oscillations (x), the frequency of the generator (f), and the velocity of the

ultrasonic waves (V) can be calculated using the given formula.

54
 2 df
V= m/s
x

After determining the velocity of the ultrasonic waves in saturated solution, the

compressibility and other acoustical parameters of the saturated solutions were

calculated using the following formulae [212].

(i) Density

ρ= w3-w1/ w2-w1 kg m-3

(ii) Viscosity

η= Пρgr4/ 8lv Nsm-2

(iii) Adiabatic compressibility

βa = 1/(V2 ρ) m2N-1

(iv) Intermolecular free length (Lf)

Inter molecular free length has been determined as

Lf = KJ (βa)1/2 m

Where K J is the temperature dependent Jacobson’s constant [213-214], is

but independent of the nature of solution.

KJ = (91.368+0.3565 T) x 10-8

Where T is the absolute temperature

(v) Internal pressure (Пi)

Пi = (bRT) [( K η)/V]1/2 [ (ρ)2/3/ M 7/6)]

Where b is the cubic packing which is assumed to be 2 for all liquids and

solutions, K is the temperature independent constant, T is the absolute temperature

and R is universal gas constant.

(vi) Relaxation time (τ)

55
 τ = (4/3) βa η seconds

(vii) Acoustic impedance (Za)

Za= V ρ (kg m-2 s-2)

The determination of the above parameters yields an insight into the molecular

interaction of species among solids and liquids. The understanding of this

information is useful in crystal growth.

CHAPTER-III

STUDIES OF TGS SINGLE CRYSTALS ADMIXTURED WITH

NITRIC ACID (TRIGLYCINE SULPHO-NITRATE)
3.1 Introduction

This chapter deals with solubility, nucleation kinetics studies, single crystal

XRD studies, FTIR studies, UV-visible spectroscopic studies and Second Harmonic

Generation (SHG) studies of pure triglycine sulphate and Triglycine sulphonitrate

(TGSN) crystals. Thermal and microhardness studies are carried out to characterize

the thermal and mechanical behaviour of the crystalline samples of pure and nitric

acid (HNO3) added TGS. The simplest mechanical property test that can be carried

out on the grown crystals is the microhardness test. The results from studies of TG/

DTA and microhardness of pure and nitric acid admixtured TGS crystals are

discussed. The values of dielectric constant, dielectric loss, AC conductivity and

activation energy of pure and nitric acid admixtured TGS crystals with various

frequencies and temperatures were determined. All the grown crystals of this work

are ferroelectrics and the results from various studies are presented and discussed in

56
 this chapter.

3.2 Synthesis and solubility

Analar Reagent (AR) grade of glycine and concentrated sulphuric acid (Merck

India) in the molar ratio 3:1 were used for synthesis of pure (undoped) Triglycine

Sulphate (TGS) salt. The required amount of sulphuric acid was diluted with

deionized water. Then the calculated amount of glycine was added to the diluted acids

to get the mother solution. The solution was heated until the synthesized salt of TGS

was obtained. During the synthesis, temperature of the solution was maintained at

50▫C in order to avoid the oxidation of the sample.

The chemical reaction for obtaining TGS salt is given below

3(NH2CH2COOH) +H2SO4 → (NH2CH2COOH) 3(H2SO4)

The purity of the synthesized salt was improved by successive recrystallization

process. To obtain nitric acid admixtured TGS salts, 10, 20, and 30 mole % of HNO 3

are added to the mother solution and the same procedure was followed as given above.

The solubility study was carried out for the samples in deionized water by

gravimetrical method [215,216]. A glass beaker with 25 ml of water was placed inside

a constant temperature bath (accuracy: ± 0.01 °C), maintained at 31 °C. TGS salt was

added in small amounts at successive stages. The addition of the salt and the stirring

were continued till the supersaturated condition is reached. The 5 ml of the saturated

solution was pipetted out and poured into a petri dish of known weight. The solvent

was completely evaporated by warming the solution at 45 °C. The amount of the salt

present in 5 ml of the solution was measured by subtracting the empty petri dish’s

weight. From this, the amount of the salt present in 100 ml of the solution was found

out. In the same manner, the amount of the salt dissolved in 100 ml at 35, 40, 45, 50,

55 and 60°C was determined. The same procedure was followed to find the solubility

57
of the nitric acid admixtured TGS samples at various temperatures. Figure 3.1 shows

that the solubility curves for pure and nitric acid admixtured TGS crystals.

Figure 3.1: Solubility curves of pure and HNO3 admixtured TGS

samples

From the graph, it is observed that the solubility of pure and nitric acid

admixtured TGS samples in water increases as the temperature increases and hence

the samples have positive temperature coefficient of solubility. The positive slope of

the solubility curve enables the growth of pure and nitric acid admixtured TGS

crystals by slow evaporation method at room temperature. The data of solubility are

useful to carry out nucleation kinetic studies.

3.3 Critical nucleation parameters and the growth of crystals

Induction period was measured for supersaturated solution of the samples using

water as the solvent at different values of supersaturation ratio and these values are

used to determine critical nucleation parameters. Using the solubility diagram, the

synthesized salt of TGS was used to prepare supersaturated aqueous solution in a

58
corning glass beaker and it was stirred continuously for about 2 hours using a

magnetic stirrer to ensure the homogeneous concentration. The nucleation cell was

loaded in a constant temperature bath (controlled to an accuracy of 0.01 oC) and

illuminated using a powerful lamp to observe the formation of nucleus. The procedure

for induction period measurement for the samples is given in the chapter-II. Two or

three trials were carried out to ascertain the correctness of the results.

Results of induction period (τ) for pure and nitric acid admixtured TGS crystals

are depicted in the figure 3.2. Induction period decreases as the supersaturation ratio

(S) increases for the samples. Since induction period decreases, the rate of nucleation

increases as the supersaturated concentration of aqueous solution is increased. When

TGS is added with nitric acid, induction period decreases and this leads to growth of

crystals faster compared to the pure TGS. From the figure 3.3, plots of ln  against

1/(ln S)2 are approximately linear which explains the classical theory of homogeneous

nucleation and the values of slope (m) were obtained by linear fit analysis for all the

four samples and the critical nucleation parameters were determined. The variations

of Gibbs free energy change, radius of critical nucleus, nucleation rate and number of

molecules in the critical nucleus with the supersaturation ratio (S) are displayed in the

figures 3.4, 3.5, 3.6 and 3.7.

The results show that the nucleation parameters such as radius of critical

nucleus, Gibbs’ free energy change and number of molecules in the critical nucleus

decrease with supersaturation and they increase when concentration of dopants is

increased. The results obtained from this work are similar to those of other crystals

done by previous workers [217,218]. The interfacial tension of the solid relative to its

solution was calculated for pure and nitric acid admixtured TGS samples and the

variation of interfacial tension as a function of the added concentration( 10 mole %,

59
20 mole % and 30 mole % ) is shown in the figure 3.8. The result of interfacial

tension shows that it increases with concentration of admixtured material. The value

of interfacial tension vary from 1.788 x10-3 to 2.0859 x10-3 J/m2

Figure 3.2: Variation of induction period () with supersaturation

ratio (S) for solution of pure and nitric acid admixtured TGS.

Figure 3.3: The plots of 1/( ln S)2 versus ln τ for solutions of pure TGS
and TGS admixtured with nitric acid.

60
 Figure 3.4: Variation of Gibbs free energy change with supersaturation
ratio (S) for solutions of pure TGS and TGS admixtured
with nitric acid.

Figure 3.5: Variation of radius of critical nucleus with supersaturation

ratio (S) for solutions of pure TGS and TGS admixtured with
nitric acid.

61
Figure 3.6: Variation of nucleation rate (J) with supersaturation ratio (S) for
solutions of pure TGS and TGS admixtured with nitric acid.

Figure3.7: Variation of number of molecules in the critical nucleus with

super saturation ratio (S) for solution of pure TGS and TGS
admixtured with nitric acid.

62
 Figure3.8: Dependence of interfacial tension with concentration of nitric acid
in the solutions of TGS samples.

Studies on nucleation kinetics of crystalline samples are carried out in order to

have the controlled nucleation rate. The number of crystals produced in the

supersaturated solution is expressed as nucleation rate i.e. the number of crystals

produced per unit volume per unit time. The variables that affect the nucleation rate

are pH, supersaturation, temperature and interfacial tension of the solution. Decrease

in induction period and increase in interfacial tension are expected to increase the

nucleation rate. With the optimized values of induction period, the growth of pure and

nitric acid admixtured TGS crystals have been grown from aqueous solutions.

Growth of pure and nitric acid admixtured TGS crystals was carried out by

solution method with slow evaporation technique at room temperature (31oC).

Synthesized and re-crystallied salt of pure and nitric acid admixtured TGS were used

to prepare the saturated solution and it was kept in the constant temperature bath. The

optimized growth parameters from the studies of nucleation kinetics were used to

grow good quality bulk single crystals. It took about 28 to 35 days to harvest the pure

and nitric acid admixtured TGS crystals. Grown crystals are shown in figure 3.9 and

the grown crystals are observed to be non-hygroscopic, colorless and transparent. It is

63
observed that the morphology (external appearance) of nitric acid admixtured TGS

crystals is slightly different when compared to that of the pure TGS crystal.

Figure 3.9: A photograph displaying (a) Pure TGS crystal, (b) TGS crystal +10
mole % of HNO3, (c) TGS crystal +20 mole % of HNO3, (d) TGS
crystal +30 mole % of HNO3
.

3.4 Structural studies

3.4.1 Single crystal X-ray diffraction analysis

X-ray diffraction analysis data were collected for the good quality single crystals

using a computer controlled Bruker-Nonius MACH3/CAD4 single crystal X-ray

diffractometer to identify the structure and to estimate the lattice parameters. The unit

cell parameters were determined using MoKradiation (λ= 0.71073 Å). It is observed

that pure and HNO3 admixtured TGS crystals crystallize in monoclinic system and the

obtained unit cell parameters are listed in Table 3.1.The pure (undoped) TGS crystal

the obtained data for in this work are found to be in good agreement with the data

reported in the literature [145,146] and slight changes of lattice parameters have been

noticed for the nitric acid admixtured TGS samples compared to that of pure TGS

crystal. The slight changes in the lattice parameters are due to incorporation of

64
admixtured material in the lattice of TGS crystal. The presence of dopants in crystal

may produce lattice strain which leads to change of unit cell parameters in the nitric

acid admixtured samples.

Table 3.1
Unit cell parameters of pure and nitric acid admixtured TGS crystals

S.No. Sample Cell parameters Volume of Unit cell

(Å)3

a = 9.392 (2) Å
b = 12.655(3) Å
1. Pure TGS crystal c = 5.727(2) Å 638.63
α = γ= 90o
β = 110.39o (2)

a = 9.437(4) Å
TGS+ 10 mole % of b = 12.645(3) Å
2. c = 5.824(2) Å 665.97
HNO3 α = γ= 90o
β = 106.50o (7)

a = 9.726(1) Å
TGS+ 20 mole % of b = 12.514(3) Å
3. c = 5.931(2) Å 689.55
HNO3 α = γ= 90o
β = 107.21o (2)

a = 9.923(4) Å
b = 12.194(3) Å
4. TGS+ 30 mole % of c = 6.123(1) Å 711.61
α = γ= 90o
HNO3 β = 106.16o (3)

65
3.4.2 Powder X-ray diffraction analysis
To confirm the crystal structure of the grown crystals and to identify the

diffracting crystal planes, powder XRD studies were carried out. The grown crystals

of pure and nitric acid admixtured TGS crystals were ground and the powder X-ray

diffraction patterns were obtained using a powder X-ray diffractometer (PAN

anlytical Model, Nickel filtered Cu Kα radiations (λ= 1.54056 Å )35 kV, 10 mA). The

samples were scanned over the required range for 2Ө (10 – 70 o). The crystalline

phase structure of the samples was identified from the crystallographic parameters

such as 2Ө, d-spacing, relative intensity and the hkl values [219]. The structural

studies of the samples were performed at room temperature (31 oC). From the X-ray

diffraction spectra, 2Ө values were read directly and the relative intensities of the

diffraction peaks were estimated. The d-spacings, corresponding to different peak

positions were calculated using the Bragg’s relation 2 d sin Ө = n λ where d is the

interplanar spacing, Ө is the Bragg’s angle, n is the order of diffraction and λ is the

wavelength of X-rays. When X-rays penetrate through the powdered sample, a

number of particles can be expected to be oriented in such a way as to satisfy the

Bragg’s condition for reflection from every possible interplanar spacing. The figures

3.10 to 3.13 present the powder XRD patterns of pure and nitric acid admixtured

crystals. The well-defined peaks at specific 2 values show high crystallinity of the

grown crystals. All the reflections of powder XRD pattern of the grown crystals were

indexed using the INDEXING software package. The XRD patterns of nitric acid

admixtured TGS crystals have the same diffraction peaks of pure TGS crystal, but

slight shift in the peaks and change in the peak intensity have been observed. A few

new peaks also have been observed in the XRD patterns of nitric acid admixtured

TGS samples. This confirms the incorporation of dopants in the lattice of TGS

66
crystals. The obtained cell parameter values for pure and nitric acid admixtured TGS

crystals by single crystal XRD studies are observed to be same. Both single crystal

XRD and powder XRD studies have been performed for the grown crystals to confirm

the values of lattice parameters.

Figure 3.10: Powder XRD pattern of pure TGS crystal

Figure 3.11: Powder XRD pattern of TGS crystal admixtured with

10mole% of nitric acid

67
Figure 3.12: Powder XRD pattern of TGS crystal admixtured with
20 mole % of nitric acid

Figure 3.13: Powder XRD pattern of TGS crystal admixtured with 30 mole%
of nitric acid.

68
 3.5 Vickers microhardness analysis

The hardness of a material is a measure of its resistance to plastic deformation.

The extent to which a material shall deform plastically under an applied stress

depends on the strength of intermolecular forces. The permanent deformation can be

achieved by indentation bending, scratching or cutting. In an ideal crystal, the

dependence is observed. This is due to normal indentation size effect (ISE) as

reported in the literature [220,221]. Vickers microhardness indentations were carried

out on the grown crystals at room temperature with the load ranging from 25 g to 100

g using Leitz pyramidal hardness tester fitted with a diamond pyramidal indenter.

Vickers microhardness number (Hv) can be calculated using the relationHv = 1.8544 P/

d2 kg/ mm2 where P is the load in kilograms, d is the diagonal length of indentation

impression in mm and 1.8544 is a constant of a geometrical factor for the diamond

pyramid [222]. Figure 3.14 represents the variation of hardness number (H v) with

load (P) for pure and nitric acid admixtured TGS crystals. From the results of

microhardness studies, it is observed that hardness number (H v) increases with load

for the grown pure and nitric acid admixtured TGS crystals. This can be explained on

the basis of depth of penetration of the indenter. When the load increases, a few

surface layers are penetrated initially and then inner surface layers are penetrated by

the indenter with increase in the load. The measured hardness is the characteristics of

these layers and the increase in the hardness number is due to the overall effect on the

surface and inner layers of the sample. For the added admixtured TGS crystals, the

hardness number is found to be increasing with increase in the concentration of

admixtured material (nitric acid). When nitric acid is added as an impurity into TGS

crystals, it may be possible that the strength of bonding improved and this leads to

69
increase of hardness in admixtured TGS crystals [223,224]. To calculate the work

hardening coefficient (n), Mayer’s law P= adn is used. Here a is constant. Plots of log

P versus log d for the samples (pure and nitric acid admixtured TGS crystals) are

displayed in the figures 3.15-3.18.

Figure 3.14: Dependence of hardness number (Hv) with loads in grams for
pure and TGS admixtured with nitric acid

70
 Figure 3.15: Plot of log P versus log d for pure TGS crystal

Figure 3.16: Plot of log P versus log d for TGS crystal admixtured
with 10 mole % of nitric acid

Figure 3.17: Plot of log P versus log d for TGS crystal admixtured with
20 mole % of nitric acid

71
 Figure 3.18: Plot of log P versus log d for TGS crystal admixtured
with 30 mole % of nitric acid

The work hardening coefficients (n) for pure TGS and nitric acid admixtured

TGS samples have been obtained from the above plots using least square fit method

and the obtained values of n are provided in the Table 3.2. It is observed from the

results that the work hardening coefficient decreases when TGS crystal is admixtured

with nitric acid.

Table 3.2

Values of work hardening coefficient (n) for pure and nitric acid
admixtured TGS crystals

Samples Work hardening coefficient (n)

Pure TGS crystal 2.65

TGS + 10 mole % of HNO3 2.54

TGS + 20 mole % of HNO3 2.46

TGS + 30 mole % of HNO3 2.37

72
Values of yield strength and stiffness constant for the samples are determined using
the following formulae
Yield strength σy = Hv / 3 Pascal
Stiffness constant C11= (Hv)7/4 Pascal

The yield strength and stiffness constant for pure TGS and nitric acid

admixtured TGS samples values are provided in the Tables 3.3- 3.6. Yield strength is

the maximum stress that can be developed in a material without causing plastic

deformation. It is observed that yield strength and stiffness constant increases with

increase of load. When nitric acid is added as an admixtured material into TGS

crystal, the values of yield strength and stiffness constant are found to be increased

and hence the grown nitric acid admixtured TGS crystals have relatively high

mechanical strength compared to the pure TGS sample. Stiffness constant gives an

idea about the measure of resistance of plastic to bending and tightness of bonding

between neighboring atoms [224,225].

Table 3.3: Yield strength and Stiffness constant for pure TGS crystal

Sample Load (grams) Yield strength (σy) Stiffness constant

x10 6 Pa x 10 15 Pa
25 158.760 1.535
Pure TGS crystal 50 190.218 2.106
75 202.990 2.360
100 218.964 2.695

Table 3.4: Yield strength and Stiffness constant for TGS crystals
admixtured with 10 mole % of HNO3.

Sample Load (g) Yield Strength (σy) Stiffness Constant

x10 6 Pa x 1015 Pa
25 166.404 1.667
TGS + 10 mole % 50 210.700 2.519
of HNO3 75 239.316 3.148
100 271.035 3.915

73
 Table 3.5: Yield strength and Stiffness constant for TGS crystals
admixtured with 20 mole % of HNO3

Sample Load(grams) Yield Strength (σy) Stiffness Constant

x 10 6 Pa x10 15 Pa
25 201.259 2.325
TGS + 20 mole % 50 239.414 3.151
of HNO3 75 270.349 3.897
100 305.270 4.821

Table 3.6: Yield strength and Stiffness constant for TGS crystals
admixtured 30 mole % of HNO3

Sample Load(g) Yield Strength (σy) Stiffness Constant

x10 6 Pa x 10 15 Pa
25 234.677 3.042
TGS + 30 mole % 50 249.671 3.391
of HNO3 75 275.058 4.094
100 319.022 5.207

3.6 Density Measurement

Floatation method was employed for the precise determination of density and this

method is sensitive to point defects and insensitive to dislocations in crystals. Xylene

(density: 0.89 g/cc) and bromoform (density: 2.89 g/cc), the rarer and denser liquids

respectively were used for the experiment. After mixing the liquids such as xylene

and bromoform in a suitable proportion in a specific gravity bottle, a small piece of

the crystal was immersed in the liquid mixture. When the sample had attained a state

of mechanical equilibrium, the density of the crystal was equal to the density of

liquid mixture. The density was calculated using the relation

ρ = (W3-W1)/ (W2-W1)

74
where W1 is the weight of the empty specific gravity bottle, W2 is the weight of the

specific gravity bottle with full of water and W 3is the weight of specific gravity

bottle full of the mixture of xylene and bromoform [226]. The values of density

of the pure and nitric acid admixtured TGS crystals are tabulated in Table 3.7. The

density of pure TGS crystal was found to be 1.653 g/cc which is in good agreement

with reported value [145,227]. It is observed that the values of density of TGS

crystals are found to increase as the concentration of dopants increases. This may be

due to incorporation of dopants in the interstitial positions of admixtured samples.

In real crystals, the concentration of interstitial positions is expected to be of the

order of 1016 – 10 /cc and these are occupied by the impurities. The density
20

variation shows very clearly that the nitric acid has entered proportionately into

TGS crystals as per the impurity concentration used for the growth of single

crystals. The density of pure TGS and nitric acid admixtured TGS crystals are also

confirmed from the crystallographic XRD data using the relation ρ = (M Z) / NV

where M is the molecular weight of the sample, Z is the number of molecules per

unit cell, N is Avogadro’s number and V is volume of the unit cell.

Table 3.7: Density of pure and nitric acid admixtured TGS crystals

Sample Density (g/cc)

Pure TGS crystal 1.653

TGS + 10 mole % of HNO3 1.666

TGS + 20 mole % of HNO3 1.674
TGS + 30 mole % of HNO3 1.680

75
3.7 FTIR studies

FTIR absorption spectra of the grown pure and nitric acid admixtured TGS

crystals in the infrared region 4500 cm-1 to 400 cm-1were recorded on a

spectrophotometer (Model: Perkin Elmer Spectrometer) using a KBr pellet

technique. The recorded FTIR spectrum of pure TGS crystal is presented in the

figure 3.19. The present vibrational spectroscopic study was carried out with a view

of obtaining an insight into the structural aspects of glycine based crystals. In order

to understand the existence of dopants and its bonding nature, the FTIR spectra of

the admixtured TGS crystals were recorded and they are presented in the figures

3.20 to 3.22. The spectra of the crystals are similar except small shifts in the peak

positions and hence the crystals are expected to preserve nearly, the same interaction

among the groups and ions. For the samples the broad band between 2200 and 3800

cm-1 in the spectra indicate stretching frequencies of superimposed O-H and NH 3+

modes. Multiple combination and overtone bands of CH 2 have been observed in the

region 2300-2500 cm-1. The NH2 asymmetric stretching vibrations appear between

3200 and 3500 cm-1 and NH2 symmetric stretching vibrations occur between 2800-

3200 cm-1.The absorption in the region 1700-1650 cm -1 is assigned to C=O stretching

of COOH group. The peaks between 1610 and 1450 cm-1 in the FTIR spectra can be

assigned to COO- vibrational mode. It is noticed here that some of NH 2 vibrations

overlap with C-N and SO4 vibrations. The strong peak region 1120-1150 cm -1 in the

samples is attributed to C-N stretching vibrations. The peaks observed at 570 and

502 cm-1 are due to NH3+ oscillation. The observed vibrational wave numbers and

their assignments for pure TGS and nitric acid admixtured TGS crystals are

tabulated in Tables 3.8-3.11. The assignments for the absorption /bands of the FTIR

spectra of the samples are provided as per the data reported in the literature

76
[228,229]. Comparing the absorption bands / peaks, it can be seen that FTIR spectra

of pure and nitric acid admixtured TGS crystals are identical with some changes.

From the spectra of doped TGS samples, the band around 3867 cm -1 to 3828 cm-1

due to stretching frequencies of superimposed O-H and NH3+ modes. The absorption

in the region 1700-1415 cm-1 is assigned to C=O stretching of COOH group. The

peaks observed at 408 and 503 cm-1 are due to C-CO bending. There is broadening or

narrowing of some absorption peaks / bands in FTIR spectra of nitric acid

admixtured TGS crystals are observed and this is due to change of bond length and

hence vibrational frequency changes and this is due to the incorporation of nitric

acid in the lattice of TGS crystals.

Figure 3.19: FTIR spectrum for pure TGS crystal

77
Figure 3.20: FTIR spectrum of TGS crystal admixtured with 10 mole % of
nitric acid

Figure 3.21: FTIR spectrum of TGS crystal admixtured with 20 mole %

of nitric acid

78
Figure 3.22: FTIR spectrum of TGS crystal admixtured with 30 mole % of
nitric acid

Table 3.8: FTIR spectral assignments for pure TGS crystals

Bands/Peaks (cm-1) Assignments

3227 NH3+ asymmetric and OH stretching

1658 NH3+ asymmetric bending
1492 COO- symmetric stretching
1415 C-N stretching
1280 C-O stretching
1202 OH-bending
945 C-C stretching
613 SO4 Scissor bending

79
Table 3.9: FTIR spectral assignments for TGS crystals admixtured with
10 mole % of nitric acid

Bands/Peaks (cm-1) Assignments

3867 N-H stretching

3828 N-H stretching
3621 C-O stretching
3336 NH3+ asymmetric and OH stretching

1788 Amide

614 NH3+ asymmetric bending

1524 SO4 Scissor bending

1389 C-H bending

1312 CH2 wagg/ Twist

1216 C-N bend of amino group

1008 C-H- out of plane deformation

798 NH2 wagging

1607 N-H symmetric bending

Table 3.11: FTIR spectral assignments for TGS crystals Admixtured with 30
mole % of nitric acid

Bands/Peaks (cm-1) Assignments

3861 N-H stretching
3507 NH3+ asymmetric and OH stretching
1830 Overtones and combinations
498 Amide
1625 C-CO bending
1539 N-H symmetric bending
1490 CH2
1329 CH2 wagg/ Twist
1109 NH3 rocking
809 NH2 wagging
615 SO4 Scissor bending
1700 NH3+ asymmetric bending

80
3.8 Energy dispersive analysis by X-rays (EDAX)

Energy Dispersive X-ray spectroscopy (EDAX) is an analytical technique used

for the elemental analysis of a sample. The EDAX spectra of pure TGS, 10, 20 and

30 mole% of nitric acid-admixtured TGS crystals are taken using a computer

controlled Scanning Electron Microscope (Model: HITACHI S-3000H) and they are

displayed in the figures.3.23-3.26. From the diagrams, it is confirmed that the

elements such as C, O, N and S were present in the sample and it is to be noted here

that the element H cannot be identified using the EDAX technique.

Figure 3.23: EDAX spectrum of pure TGS crystal

81
Figure 3.24: EDAX spectrum of 10 mole % of HNO3 admixtured TGS
crystal

Figure 3.25: EDAX spectrum of 20 mole % of HNO3 admixtured TGS

crystal

82
 Figure 3.26: EDAX spectrum of 30 mole % of HNO3 admixtured TGS
crystal

3.9 Linear optical constants

The linear optical constants such linear absorption coefficient, extinction

coefficient, refractive index and reflectance etc have been determined from UV-

visible transmittance spectrum of pure TGS and nitric acid admixtured TGS crystals.

The optical transmission spectrum of pure TGS and nitric acid admixtured TGS

crystals were recorded using a UV-vis-NIR spectrophotometer (Lamda 35 model) in

the range of 190-1100 nm. A good quality crystal of pure TGS and nitric acid

admixtured TGS crystals with a thickness of 2 mm were used in this study. The

obtained transmittance spectrum of pure and nitric acid admixtured TGS crystal is

shown in figure 3.27. It shows that the lower cut-off wavelength at 236 nm and there

is good transmission in the entire visible region that corresponds to the suitability for

harmonic generations [230]. The lower cut-off absorption is an encouraging optical

property seen in pure and nitric acid admixtured TGS crystals and is of vital

83
importance for ferroelectric materials. It is observed that the magnitudes of energy

band gap for pure and nitric acid admixtured TGS crystals are the same. Using the

formula Eg = 1240 / 236  (nm), the band gap is calculated to be 5.254 eV. The

optical absorption coefficient of photon energy helps to study the band structure and

explain the type of transition of electrons. The optical absorption coefficient (α) was

calculated using the following formula [ 231].

1
2 .303 log ( )
T
α=
d

where T is the transmittance and d is the thickness of the crystal in mm.

Figure 3.27: UV-visible transmittance spectra for pure and nitric acid
admixtured TGS crystals

The extinction coefficient (K) and reflectancet (R) are calculated using the

following relations [231-232]

84
 λα
K= 4 π

1 ± √1−e(−αd ) +e (αd )
R= ( −αd )
1+ e

The relation between linear refractive index (n) and reflectance ( R ) is given by

− ( R+ 1 ) ± √ (−3 R + 10 R−3 ) ( R+1 ) ± √( 3 R 2+10 R−3 )

2
n= n=
2 ( R−1 ) 2(1−R)
− ( R+ 1 ) ± √ (−3 R + 10 R−3 )
2
n=
2(1−R)

The energy dependence of absorption coefficient suggests the occurrence of

direct band gap of the crystal obeying the following relation

∝hν= A √(hν−E g)

where Eg is the optical band gap energy of the crystal, h is the Planck’s constant, γ is

the frequency and A is a constant. The plot of variation of (αℎυ) 2 versus ℎυ is

(Tauc’s plot) shown in the figure 3.28 , and the band gap energy is calculated by

extrapolation of linear part. The bandgap energy is found to be 5.25 eV. Using the

above equations the values of absorbtion coefficient, extinction coefficient,

reflectance and refractive index for pure and nitric acid admixtured TGS crystal were

determined. The variations of extinction coefficient, absorbtion, reflectance and

refractive index with wavelength are shown in the figures 3.29, 3.30, 3.31 and 3.32.

The low absorption, low reflectance and low refractive index of pure and nitric acid

admixtured TGS crystals in the absorption spectrum region makes the samples that

are the prominent materiasl for antireflection coating in solar thermal devices and

nonlinear applications [231].

85
Figure 3.28 :Plots of ( αhν)2 versus hυ for pure and nitric acid admixtured TGS
crystals

Figure 3.29: Variation of extinction coefficient with wavelength for pure and
nitric acid admixtured TGS crystals

86
 Figure 3.30: Plots of absorbance versus wavelength for pure and nitric
acid admixtured TGS crystals.

Figure 3.31: Plots of reflectance versus wavelength for pure and nitric acid
admixtured TGS crystals

87
Figure 3.32 : Variation of refractive index with wave length for pure and nitric
acid admixtured TGS crystals.

From results, compared to pure TGS crystal, due to impurity (nitric acid)

admixtured TGS crystals have high reflectance, absorbance and extinction coefficient.

The internal efficiency of the optical devices depends upon the absorption coefficient

and reflectance. Hence by tailoring the absorption coefficient and tuning the band gap

of the material, we can achieve the desired material which is suitable for fabricating

various layers of the optoelectronic devices as per our requirements. From the figure.

3.32 the refractive index decreases with increasing wavelength. Due to presence of

impurities, the admixtured TGS crystals have high refractive index than that of pure

TGS crystal.

3.10 Second Harmonic Generation (SHG) studies

The second harmonic generation test for the grown pure and nitric acid

admixtured TGS samples was performed by the powder technique of Kurtz and

88
Perry. The grown crystals were powdered and a high intensity Nd: YAG laser light

(λ=1064 nm) with a pulse duration of 8 ns was passed through the powdered samples.

There is no emission of green light and it shows that the materials have no SHG.

3.11 Dielectric analysis

3.11.1 Dielectric constant and dielectric loss
The dielectric constant (ε r) and the dielectric loss (tan δ) of the samples were

measured using LCR meter (Agilent 4284A) in the frequency region 1 kHz and 10

kHz. Defect free and transparent crystals were selected and used for the electrical

measurement. For good conduction, opposite faces of the sample crystals were

coated with good quality graphite. The dielectric constant and the dielectric loss

were estimated for varying frequencies under different temperature slots from 35

to 90 °C. The temperature dependence of dielectric constant (εr) obtained for the

grown crystals for 1 kHz frequency is provided in figure 3.33(a) and for 10 kHz

frequency in figure 3.33(b). The temperature dependence of dielectric loss (tan δ)

obtained for the frequency 1 kHz is presented in figure 3.34(a) and for 10 kHz in

figure 3.34 (b). From the graphs, it is observed that as the temperature increases,

the value of dielectric constant and dielectric loss increase for both pure and nitric

acid admixtured TGS crystals. Low values of dielectric loss indicate that the grown

crystals are of good quality dielectric materials

89
Figure 3.33 (a) : Variation of dielectric constant with temperature for pure
and nitric acid admixtured TGS crystals at 1 kHz.

Figure 3.33 (b): Variation of dielectric constant with temperature for

pure and nitric acid admixtured TGS crystals at 10 kHz.

90
 Figure 3.34(a): Variation of dielectric loss with temperature for pure and
nitric acid admixtured TGS crystals at 1 kHz

Figure 3.34(b): Variation of dielectric loss with temperature for pure and
nitric acid admixtured TGS crystals at 10 kHz

The dielectric constant of a material is generally composed of four types of

contributions, viz. ionic, electronic, and orientational and space charge polarizations.

91
At low frequencies, dielectric constant of the material is mainly due to the

contribution of space charge polarization. The dielectric constant and the dielectric

loss values increase with the increase in temperature for both the frequencies 1 kHz

and 10 kHz upto the transition temperature (49.5 o C for pure TGS crystal) and

then decrease. Above Tc, the dielectric constant decreases and obeys Curie-Weiss

law. The nitric acid admixtured TGS crystal has higher dielectric constant and loss

values compared to that of pure TGS crystal. Increase in dielectric constant for nitric

acid admixtured TGS crystal at transition temperature attributed to free charge carriers

created by the dopant. The Curie point T c for the samples was not found to be the

same. A small shift in Tc (51oC) is found for nitric acid admixtured TGS crystals.

Variation of εr with temperature is generally attributed to the crystal expansion, the

electronic and ionic polarizations and the presence of impurities and crystal defects. It

suggests that pure and nitric acid admixtured TGS crystals seem to contain molecules

of varying relaxation times. At higher frequencies, the values of dielectric constant

and loss are low because molecules of larger relaxation times may not be able to

respond to these higher frequencies [235].

3.11.2 AC conductivity

AC conductivity (σac) of the grown crystals for different frequencies can be

determined using the relation

σac = 2Пf εr ε0 tan δ

where f is the frequency of AC supply, εr is the dielectric constant, εo is the

permittivity of free space, tan δ is the dielectric loss. Using the measured values of

dielectric constant and loss, AC conductivity was calculated. AC conductivity values

obtained for the grown crystals for 1 kHz frequency and for 10 kHz frequency are

92
provided in figures 3.35(a) and 3.35(b). Here also the values of AC conductivity are

observed to be maximum at the Curie point and noticed to be more when TGS

crystals are admixtured with nitric acid.

Figure 3.35 (a): Variation of AC conductivity with temperature for pure

and nitric acid admixtured TGS crystals at 1 kHz

Figure 3.35 (b): Variation of AC conductivity with temperature for pure

and nitric acid admixtured TGS crystals at 10 kHz

93
 3.11.3 Activation Energy

The AC conductivity values are fitted in the equation σ ac = σo exp (-E/kT) where σo

is a constant which depends upon the type of the sample, E is the activation energy, k is

the Boltzmann’s constant and T is the absolute temperature. A graph is drawn between

ln σac and 1/T which gives a straight line. The slope of the straight line is equal to E/k

from which the activation energy (E) can be calculated. Ferroelectric crystals are

concerned, the ferroelectric region is important and hence activation energy is calculated

for the temperature region below the Curie point (Tc). Plots of ln σac versus 1000/T for

grown crystals are shown in the figures 3.36 (a) and 3.36 (b). The slope values were

obtained in the temperature range 30-50 oC by least fit square method. The obtained

values of activation energy for the grown pure and nitric acid admixtured TGS crystals

in the ferroelectric region are provided in the Table 3.12. From the results, it is noticed

that the activation energy values decrease as the concentration of the dopant

increases. The decrease in activation energy values for the nitric acid admixtured

TGS crystals may be due to large carrier concentration in the samples.

94
Figure 3.36 (a): Plots of ln σac versus 1000/T for pure and nitric acid admixtured
TGS crystals at1kHz

Figure 3.36 (b): Plots of ln σac versus 1000/T for pure and nitric acid
admixtured TGS crystals at 10 kHz

Table 3.12: Values of activation energy for pure and nitric acid admixtured
TGS crystals

Frequency Values of activation energy (eV)

Pure TGS+10 mole % TGS+20 mole % TGS+30 mole %

TGS
of HNO3 of HNO3 of HNO3

1 kHz 0.701 0.6828 0.5020 0.4628

1

10 kHz 1.06 0.6607 0.5547 0.5387

95
3.12 Thermal studies
TG and DTA thermograms of the pure TGS and nitric acid admixtured TGS

crystals are displayed in figures 3.37, 3.38, 3.39 and 3.40. The samples were heated

in a crucible between 30 °C and 1010 °C at a heating rate of 20 °C/minute in

nitrogen atmosphere. The weights of the samples used were 3.0540 mg for

pure TGS, 5.0120 mg for 10 mole % of nitric acid admixtured TGS, 2.1087 mg for

20 mole % of nitric acid admixtured TGS, and 3.135 mg for 30 mole % of nitric

acid admixtured TGS. The TG curve shows the corresponding weight losses at

various stages. The initial mass of the sample of pure TGS used was 3.0540 mg at

30 oC but final mass left out after the experiment was only 0.06153% (0.0014154

mg) at 1000 oC (Fig. 3.37). The TG curve shows that there was weight loss of about

0.36 % in the temperature up to 113.33 oC. The maximum weight loss (53.41%) is

observed in the temperature range 219.34 °C - 250.54°C. From DTA curve, it is

observed that there is one endothermic peak at 244.47°C which represents the

decomposition of pure TGS. From this, it is concluded that the crystal decomposes

only at 244.47 °C. The sharp endothermic peak shows good degree of

crystallinity of the sample. For TGS crystals admixtured with nitric acid (10, 20 and

30 mole %), TG/DTA curves (figures 3.38, 3.39 and 3.40) show that they are

thermally stable upto 280C. The TG curves show that there is a maximum weight

loss in the temperature range 208 – 270 C. Beyond 600 C, the weight loss is very

little and the residue is found very small at the end of the experiment. DTA thermal

plots with respective exothermic peaks in the range 350-600 oC may be due to

liberation of volatile substances probably ammonia and / or carbon dioxide. It is

concluded from thermal studies that the sample crystals show the same thermal

96
behaviour except that there is a slight shift of endothermic transitions due to

incorporation of dopants as impurities in the lattice of TGS crystals. The values of

decomposition point of pure and nitric acid admixtured TGS crystals are provided

in Table 3.13.

Figure 3.37: TG/DTA thermograms of pure TGS crystal

97
 Figure 3.38: TG/DTA thermograms of 10 mole % of HNO3 admixtured TGS
crystal

Figure 3.39: TG/DTA thermograms of 20 mole % of HNO3 admixtured TGS

crystal

98
 Figure 3.40: TG/DTA thermograms of 30 mole % of HNO3 admixtured TGS
crystal

Table 3.13: Decomposition point of pure and HNO 3 admixtured TGS

crystals

Samples o
Decomposition point ( C)

Pure TGS 244.27

TGS + 10 mole % of HNO3 248.49

TGS+ 20 mole % of HNO3 263.61
TGS+30 mole % of HNO3 280.63

3.13 Ultrasonic studies

Ultrasonic studies were carried out for the samples using an ultrasonic

interferometer. The sample cell is filled with the aqueous saturated solution. The

procedure to find the acoustical parameters of the samples is given in the chapter- II.

The formulae used to determine the ultrasonic velocity, density, viscosity,

compressibility; intermolecular free length, internal pressure, relaxation time and

99
acoustic impedance could be found in the chapter-II. The values of acoustical

parameters such as velocity, density and viscosity of pure and nitric acid (10, 20 and

30 mol %) admixtured TGS samples at room temperature are provided in the Table

3.14. The values of other acoustical parameters are provided in the Table 3.15.

From the studies, it is observed that the ultrasonic velocity, density, viscosity,

internal pressure, relaxation time and acoustic impedance of the saturated solutions of

nitric acid admixtured TGS samples are found to be higher than that of the pure

saturated solution of TGS. But adiabatic compressibility and intermolecular free

length of pure TGS sample are higher than that of admixtured samples. The measured

acoustical parameters are useful in understanding the molecular interaction and the

formation of crystal nuclei in the saturated solutions [236].

Table 3.14: Values of ultrasonic velocity, viscosity and density of saturated

solutions of pure and nitric acid admixtured TGS samples

Ultrasonic Density ( ρ) Viscosity ( η)

Sample Velocity Kg m-3 (10-3)
(V) Nsm-2
m/s

Pure TGS 1615 1658 1.388

TGS + 10 mol% of 1619 1671 2.9144

HNO3

TGS+ 20 mol% of 1623 1679 6.312

HNO3

TGS+30 mol% of 1628 1688 10.034

HNO3

Table 3.14: Values of compressibility, free length, internal pressure, relaxation

100
 time and acoustic impedance of saturated solutions of pure and
nitric acid admixtured TGS samples

βa Lf Пi τ Za

Sample 10-10 10-10 106 10 -12 106

m2N-1 m (pascal (s) (kgm-2s-2)

second)

Pure TGS 2.3124 0.3031 49.136 0.4282 2.6676

TGS + 10 2.2831 0.3012 74.518 0.8871 2.7053

mole % of
HNO3

TGS+ 20 2.2610 0.2998 111.234 1.9029 2.7250

mole % of
HNO3

TGS+30 2.2352 0.2980 140.031 2.9904 2.7480

mole % of
HNO3 .

CHAPTER-IV

101
 CHARACTERIZATION OF TRIGLYCINE SULPHO-PICRATE
(TGSPi) CRYSTALS
4.1 Introduction

The growth of Triglycine sulpho-picrate crystals was carried out by solution

method. The grown crystals were subjected to various studies such as single crystal

XRD and powder XRD studies, FTIR studies, microhardness, thermal studies, second

harmonic studies, ultrasonic studies, dielectric studies, UV-vis studies etc. The

obtained data are discussed in this chapter.

4.2 Experimental procedure

4.2.1 Synthesis

T he synthesis for pure TGS sample is given in the chapter III. To prepare 10

mole % of picric acid (C6H3N3O7) added TGS sample, TGS salt and picric acid were

taken in 0.9: 0.1 molar ratio. The calculated reactants were dissolved in deionized

water, stirred well, heated at 50 oC to get the picric acid admixtured TGS salt. Similar

procedure was followed to prepare 20 mole % and 30 mole % of picric acid

admixtured TGS samples.

4.2.2 Solubility

The solubility study was carried out for the picric acid added TGS samples

in deionized water by gravimetrical method as given in the chapter-III figure 4.1

shows that the solubility curves for pure and picric acid admixtured TGS crystals.

From the results, it is noticed that the solubility of pure and picric acid admixtured

TGS samples in water increases as the temperature increases [237]. It is observed that

the picric acid admixtured TGS samples have more solubility than that of pure TGS

sample and it is due to addition of picric acid into the solutions of TGS.

102
 Figure 4.1: Solubility curves of pure and picric acid admixtured TGS
crystals
.
4.2.3 Nucleation Kinetics

Using the values of induction period, the critical nucleation parameters of the

samples were determined. The values of induction period were measured by

isothermal method [238,239]. The procedure for the measurement of induction period

for the samples is given in the chapter-II. The variation of induction period (τ) with

supersaturation ratio (S) for pure and picric acid admixtured TGS crystals are depicted

in the figure 4.2.Induction period () decreases as the supersaturation ratio (S)

increases for the samples. From the figure 4.3, plots of ln  against 1/( ln S)2, the

values of slope were obtained for the samples and the critical nucleation parameters

were determined. The obtained values of Gibbs free energy change, radius of critical

nucleus, number of molecules in the critical nucleus and nucleation rate are

summarized in Table 4.1. From the results, it is observed that the values of interfacial

tension and other nucleation parameters increase when TGS are admixtured with

picric acid. But the nucleation rate is found to be decreasing when TGS are added

with picric acid. The data of nucleation parameters are used to understand the

103
nucleation phenomena and to grow good quality crystals [239,240].

Figure 4.2: Variation of induction period () with supersaturation ratio(S)

for solutions of pure and picric acid admixtured TGS

Figure 4.3: The plots of 1/( ln S)2 versus ln τ for solutions of pure TGS
and TGS admixtured with picric acid.
Table 4.1: Summary of critical nucleation parameters for pure and picric
acid admixtured TGS samples.

104
 S Τ ∆G*(10-21) r*(10-9) n σ (10-3) J x1024
(J) m
Sample ( sec) J/m2 (Nuclei/sec/
volume)

1.3 7300 9.6553 1.0686 16 1.300

1.35 6150 7.3793 0.9347 10 1.722

Pure TGS 1.4 5246 5.8704 0.8337 7 1.788 2.462

1.45 3950 4.8139 0.7549 5 3.174

1.5 2825 4.0425 0.6918 4 3.815

1.3 6900 11.7407 1.1445 21 1.167

1.35 5748 9.1519 1.1093 15 1.472

TGS+10 1.4 4957 7.0984 0.9456 11 2.0566 2.165

mol% of
C6H3N3O7 1.45 3413 6.3608 0.8615 9 3.020

1.5 2612 5.8223 0.7991 8 3.560

1.3 6322 14.6714 1.1853 26 1.127

1.35 5256 11.8632 1.1546 20 1.234

TGS+20 1.4 4607 10.6642 1.1224 16 2.1669 2.011

mol% of
C6H3N3O7 1.45 2928 9.8248 0.9987 12 2.841

1.5 2045 8.212 0.8764 10 3.312

1.3 5554 18.9994 1.289 31 0.992

1.35 4323 15.8783 1.1942 26 2.3859 1.112

TGS+30 1.4 3547 13.4717 1.1647 21 1.847

mol% of
C6H3N3O7 1.45 2414 11.487 1.1215 17 2.514

1.5 1412 10.768 1.1114 13 2.994

4.2.4 Growth of crystals

105
 Doping on pure crystals is interesting because of the fact that the dopants may

significantly change the properties. Hence different concentrations of dopants have

been incorporated into the undoped (pure) crystal of TGS and the various studies have

been carried out. In accordance with the solubility data, saturated solutions of the

synthesized salts of pure and picric acid admixtured TGS were prepared separately.

The solutions were constantly stirred for about 2 hours using a hot plate magnetic

stirrer and were filtered using 4-micro Whatmann filter papers. Then filtered solutions

were kept in beakers covered with porus papers and kept in a dust free atmosphere. To

grow big size crystals, seed technique has been used. During the growth, very small

crystals appeared at first which then grew bigger on slow evaporation. Constant

temperature bath was also used to maintain the temperature constant. Several trials

were tried to get good quality crystals. The grown crystals of picric acid admixtured

TGS are displayed in the figures 4.4 to 4.6. The Table 4.2 gives the dimensions and

the growth period for picric acid admixtured TGS crystals.

Figure 4.4: TGS crystal admixtured with picric acid (10 mole %)

106
 Figure 4.5: TGS crystal admixtured with picric acid (20 mole %)

Figure 4.6: TGS crystal admixtured with picric acid (30 mole %)

The grown crystals are found to be stable, non-hygroscopic, yellow coloured and

transparent. It is noticed that appearance of yellow colour gets increased when the

concentration of picric acid is increased in the solution of TGS. The reported literature

indicates that dopants act as immobile impurities that are usually adsorbed at the

terrace of the crystal during the growth. Adsorption of dopants on the surface of the

crystal takes place during the growth and hence the dopants have been introduced

into the lattice of the TGS. It is possible that the presence of dopants may lead to

change of physical and other properties of the TGS crystals [241,242].

107
Table 4.2: Size and growth period for the grown crystals

Sample Dimensions (mm3) Growth period

TGS+10 mole % of C6H3N3O7 20x18x12 25-28 Days

TGS+20 mole % of C6H3N3O7 18x17x13 28-32 Days

TGS+30 mole % C6H3N3O7 21x 19 x 14 32-35 Days

4.3 Results and discussion

4.3.1 Single crystal XRD studies

The single crystal XRD analysis is non-destructive and gives information about

the crystal structure of the sample. The grown crystals were subjected to single XRD

studies and the data were collected using Bruker-Nonius MACH3/CAD4 single X-

rays diffractomer. The obtained unit cell parameters by single crystal XRD method

for picric acid admixtured TGS crystals are given in Table 4.3. The results in the

Table 4.3 show that the picric acid admixtured TGS crystals crystallize in monoclinic

structure. The space group for picric acid admixtured TGS crystals is P21. The number

of molecules per unit cell was found to be 2. The results show that there is a

systematic variation of volume of unit cell when TGS crystal is admixtured with 10

mole %, 20 mole %, and 30 mole % of picric acid. However the differences in the

lattice volumes for picric acid admixtured crystals observed in the present study are

very small to create any lattice distortion in the host crystals. In addition, the values of

lattice constants are found to be inconsistent in some cases and it may be due to the

irregular growth and essentially due to the non-uniform adsorption of the impurity on

the growing faces of crystals [243-245].

108
Table 4.3: Unit cell parameters of picric acid admixtured TGS crystals

S.No. Samples Cell parameters Volume of unit cell

(Å)3

a = 9.650(13) Å
b = 11.151(4) Å
TGS+ 10 mole % of c = 6.242(2) Å
1. α = γ= 90o 653.68
C6H3N3O7 β = 103.29o

a = 9.146(1) Å
TGS+ 20 mole % of b = 12.716(3) Å
2. c = 5.921(2) Å 663.57
C6H3N3O7 α = γ= 90o
β = 105.50 o

a = 9.352(2) Å
TGS+ 30 mole % of b = 12.948 (1) Å
3. c = 5.969(3) Å 690.34
C6H3N3O7 α = γ= 90o
β = 107.230

4.3.2 Powder XRD studies

The grown picric acid admixtured TGS crystals have been crushed into

uniform fine powder and subjected to powder X-ray diffraction analysis using

PANalytical model nickel filtered CuK  radiation ( λ = 1.54056 Å ) to identify the

reflection planes. All the reflections of powder XRD pattern of the grown crystal were

109
indexed using the INDEXING and TREOR software packages following the

procedure of Lipson and Steeple [219]. The indexed powder X-ray diffraction patterns

of the grown crystals are given in figures 4.7 to 4.9.

Figure 4.7: Powder XRD pattern of TGS crystals admixtured with 10 mole%
of picric acid

110
 Figure 4.8: Powder XRD pattern of TGS crystals admixtured with
20 mole of % picric acid

Figure 4.9: Powder XRD pattern of TGS crystals admixtured with

30 mole of % picric acid

The powder XRD studies of the grown crystals give ideas as crystalline phase

and X-ray reflection planes. Also this method is used obtain lattice parameters of

powdered samples. The sharp peaks and low full-width and half maximum of the

powder XRD patterns of this work confirm that the crystallinity of the grown crystals

are good. It is found that the reflection lines of the pure and picric acid admixtured

TGS crystals correlate well with those observed in the individual parent compounds

with a slight shift in Bragg’s angle. The variation in the peak intensities of the various

diffraction patterns on changing the concentration of doping was also observed. The

above mentioned changes are due to the incorporation of dopants in the interstitials of

the host crystals. The unit cell parameters were also found using the powder XRD

data and UNITCELL software package and found that the values are observed to be

coinciding with the single crystal XRD data.

111
4.3.3 Fourier Transform Infrared (FTIR) analysis
The IR spectrum contains the entire information about the molecular

structure of the grown sample. The recorded FTIR spectra of picric acid admixtured

TGS crystals are presented in figures 4.10 to 4.12. From the FTIR spectra, it is

observed that there are some new peaks appeared compared to that of pure TGS

crystal (Fig.3.19) and this indicates that picric acid has entered into the lattice of host

TGS samples. The observed vibrational wave numbers and their assignments for

picric acid admixtured TGS crystals are shown in Table 4.4 to 4.6. These results

are used to identify the various functional groups such as NH3+, CH, COO-, SO4,

C-O, and C-S-N etc of the samples.

Figure 4.10: FTIR spectrum of TGS crystals admixtured with 10 mole % of

picric acid

112
Figure 4.11: FTIR spectrum of TGS crystals admixtured with 20 mole % of
picric acid

Figure 4.12: FTIR spectrum of TGS crystals admixtured with 30 mole %

of picric acid

113
Table 4.4: FTIR spectral assignments for TGS crystals admixtured with 10
mole % of picric acid

Bands/Peaks (cm-1) Assignments

3396 NH3+ asymmetric and OH stretching

2751 NH2 symmetric stretching

2678 +
NH3 stretching vibration

2283 Stretching of CH3 vibration

1678 +
NH3 asymmetric bending

1370 -
COO symmetric stretching

1328 C-O stretching

1115 +
NH3 rocking

994 SO4 stretching vibration

550 C-S-N bending vibrations

Table 4.5: FTIR spectral assignments for TGS crystals admixtured with 20
mole % of Picric acid
Bands/Peaks (cm-1) Assignments

3413 NH3+ asymmetric and OH stretching

2885 NH2 symmetric stretching

2769 CH2 stretching

2351 Absorption due to – CH2 grouping

1684 NH3 + asymmetric bending

1389 COO- symmetric stretching

1309 C-O stretching
1124 NH3+ rocking

995 SO4 stretching vibration

592 C-S-N bending vibrations

114
Table 4.6: FTIR spectral assignments for TGS crystals admixtured with 30
mole % of Picric acid

Bands/Peaks (cm-1) Assignments

3500 NH3+ asymmetric and OH stretching

3200 OH vibration

2928 NH3+ symmetric stretching

2800 NH2 symmetric stretching

2375 Absorption due to – CH2 grouping

1684 NH3 + asymmetric bending

1395 COO- symmetric stretching

1320 C-O stretching

1105 NH3+ rocking

1010 SO4 stretching vibration

602 C-S-N bending vibrations

4.3.4 Density Measurement

The density values of picric acid TGS crystals were measured by floatation

technique. The calculated values of density of picric acid admixtured TGS crystals are

given in the Table 4.7. From the results it is noticed that the density increases as the

concentration of picric acid is more in the samples.

Table 4.7: Density of pure and picric acid admixtured TGS crystals

Sample Density (g/cc)

Pure TGS crystal 1.653

TGS + 10 mole % of picric acid 1.662

TGS + 20 mole % of picric acid 1.672
TGS + 30 mole % of picric acid 1.679

115
4.3.5 Analysis of microhardness

The microhardness studies were carried out to determine the mechanical strength

of the grown crystal. In an ideal crystal, the hardness value should be independent of

applied load. The static indentations were made on the (010) face of the crystal by

varying the load from 25-100 g at room temperature. The variation of microhardness

number with different loads applied to the samples are provided in the figure 4.13,

and it is noticed that Vickers hardness number (Hv) increases with the applied load

satisfying the reverse indentation size effect [246,247]. From the results, it is observed

that the hardness number of TGS crystal increases when it is added with picric acid.

The incorporation of picric acid in the lattice of admixtured crystal may enhance the

strength of bonding in the host material and hence hardness number increases. Values

of yield strength and stiffness constant for the samples are determined using the

formulae as given in the chapter-III. The yield strength and stiffness constant for

picric acid admixtured TGS samples values are provided in the Tables 4.8-4.10. From

the results, it observed that values of yield strength and stiffness constant increase

when picric acid is added into TGS crystals.

Figure 4.13: Dependence of hardness number (Hv) with loads in grams for pure
and picric acid admixtured TGS crystals

116
 `
Table 4.8: Yield strength and Stiffness constant for TGS crystals
admixtured with 10 mole % of picric acid

Sample Load (g) Yield strength (σy) Stiffness constant

x10 6 Pa x 1015 Pa
25 181.92 1.948
TGS + 10 mole % 50 210.17 2.508
of picric acid 75 219.25 2.701
100 234.84 3.046

Table 4.9: Yield strength and Stiffness constant for TGS crystals
admixtured with 20 mole % of picric acid

Sample Load(grams) Yield strength (σy) Stiffness constant

x 10 6 Pa x10 15 Pa
25 197.83 2.256
TGS + 20 mole % 50 232.88 3.002
of picric acid 75 250.89 3.420
100 268.74 3.857

Table 4.10: Yield strength and Stiffness constant for TGS crystals
admixtured 30 mole % of picric acid

Sample Load(g) Yield strength (σy) Stiffness constant

x10 6 Pa x 10 15 Pa
25 228.92 2.913
TGS + 30 mole % 50 243.10 3.236
of picric acid 75 279.00 4.118
100 323.20 5.327

4.3.6 UV-visible spectroscopic studies

UV-visible absorption spectra are attributed to a process in which the outer

electrons of atoms or molecules absorb radiant energy and undergo transitions to

higher energy levels. These electronic transitions are quantized and depend on the

electronic structure of the absorber. The wavelength at which and ultraviolet

absorbance maximum is found, depends upon the magnitude of the energy involved

117
for a specific electronic transition. The amount of absorption depends upon the

wavelength of the radiation and the structure of the compound. The absorption of

radiation is due to subtraction of energy from the beam of radiation when electrons in

orbitals of lower energy (valance electrons) get excited into orbitals of higher energy.

The absorption of radiation causes the molecule or atom to be in an excited state.

Frequency of absorbed radiation provides a means of characterizing the constituents

of a sample of matter [248]. For this purpose transmission is plotted against

wavelength or frequency. The UV-visible transmittance spectra for picric acid

admixtured TGS crystals are presented in figure 4.14. The transmittance of host TGS

crystals are observed to be decreased when picric acid is added into TGS crystal. The

results show that the cut-off wavelength slightly changes when TGS crystals are

admixtured with picric acid and the values are provided in the Table 4.13. The Tauc’s

plots for the samples are shown in the figure 4.15. Using the Tauc’s plots, the optical

band gap was calculated and are given the Table 4.11.

Figure 4.14: UV-visible transmittance spectra for pure and picric acid
admixtured TGS crystals

118
 Table 4.11: Cut-off wavelength and band gap values for pure and picric acid
admixtured TGS crystals

Sample Cut-off Band gap(eV)

wavelength(nm)
Pure TGS crystal 236 5.27
TGS+10 mole% of 240 5.16
picric acid
TGS+20 mole% of 240 5.16
picric acid
TGS+30 mole% of 240 5.16
picric acid

Linear optical constants such as reflectance, extinction coefficient and

refractive index are calculated as the procedure given in the chapter-III. Figures

4.16, 4.17, 4.18 and 4.19 show variations of extinction coefficient, reflectance,

absorbance and refractive index with wavelength for pure and picric acid

admixtured TGS crystals. It is noticed from the results that the values of extinction

coefficient, reflectance and refractive index increase when TGS crystals are added

with picric acid.

Figure 4.15: Plot of ( αhν)2 versus hυ for pure and picric acid admixtured
TGS crystals.

119
Figure 4.16: Variation of extinction coefficient with wavelength for pure and
picric acid admixtured TGS crystals

Figure 4.17 :Plots of reflectance versus wavelength for pure and picric acid
admixtured TGS crystals

120
 Figure 4.18:Plots of absorbance versus wavelength for pure and picric
acid admixtured TGS crystals.

Figure 4.19 : Variation of refractive index with wave length for pure and picric
acid admixtured TGS crystals.

4.3.7 EDAX analysis

121
 The EDAX spectra of the picric acid TGS samples are given in the figures 4.20

to 4.22. These spectra were taken using a scanning electron microscope (Model:

HITACHI S-3000H) at STIC, CUSAT, Cochin. From the results, it is confirmed

that the elements such as C, O, S and N were present in the samples.

Figure 4.20: EDAX spectrum of 10 mole% of picric acid admixtured TGS

crystal

Figure 4.21: EDAX spectrum of 20 mole% of picric acid admixtured TGS

crystal

122
 Figure 4.22: EDAX spectrum of 30 mole % of picric acid admixtured TGS
Crystal

4.3.8 NLO Studies

Second order nonlinear optical studies were carried out by Kurtz – Perry powder

technique. A high intensity Nd: YAG laser (λ=1064 nm) with a pulse duration of 8 ns

was passed through the powdered sample. The SHG was confirmed by the emission

of green radiation which was detected by a photomultiplier tube. In this experiment,

Potassium Dihydrogen Phosphate (KDP) was used as the reference material. The

values of relative SHG efficiency of pure and picric acid admixtured TGS crystals

with reference to KDP are tabulated in Table 4.12

Table 4.12: Values of relative SHG efficiency of pure and picric acid
admixtured TGS samples

Relative SHG
Sample
efficiency
Pure TGS crystal Nil
TGS+10 mol % of picric acid 0.64
TGS+20 mol % of picric acid 0.93
TGS+30 mol % of picric acid 1.10

123
 The powder SHG test confirms the NLO property of picric acid admixtured TGS

crystals. The pure TGS crystal has nil SHG efficiency. When TGS crystals are

admixtured with organic (picric acid) dopants, SHG is observed and it is found that

SHG efficiency relative to that of KDP increases with increase in concentration of

picric acid. The results show that picric acid admixtured TGS crystals are the second

harmonic generators and are observed to be better candidates for NLO applications.

4.3.9 Dielectric studies

4.3.9.1 Dielectric constant and dielectric loss

The dielectric parameters depend on the frequency of the A C voltage applied

across the material. The sample was electroded on either side with good quality

graphite to make it behave like parallel plate capacitor. Dielectric measurement

was carried out using a precision (Model:4284A) LCR meter in the frequency

range 1 kHz and 10 kHz. The temperature dependence of dielectric constant (εr)

obtained for the grown crystals for 1 kHz frequency is provided in figure 4.23 (a) and

for 10 kHz frequency in figure 4.23 (b). The temperature dependence of dielectric

loss (tan δ) for the picric acid added TGS crystals are shown in the figures 4.24 (a)

and 4.24 (b). From the graphs, it is observed that as the temperature increases,

the value of dielectric constant and dielectric loss increase for pure and picric acid

admixtured TGS crystals. The dielectric constant and the dielectric loss values

increase with the increase in temperature upto the transition temperature and then

they decrease. The picric acid admixtured TGS crystal has higher dielectric

constant and loss values compared to that of pure TGS crystal. The Curie point T c for

the picric acid admixtured TGS crystals is observed to be 50 oC and it is slightly

more than that of pure TGS crystal. Using the values of dielectric constant and loss

124
factor, AC conductivity was calculated and the variations of AC conductivity with

the temperature are presented in the figures 4.25(a) and 4.25 (b). The activation

energy was determined in the ferroelectric region using the plots (Fig. 4.26(a) and

4.26(b)) of ln (σac) with 1000/T for 1kHz and for 10 kHz frequency. The obtained

values of activation energy for the samples are provided in the Table 4.13. It is

observed from the results that the conductivity increases and the activation energy

decreases when TGS crystals are added with picric acid.

Figure 4.23 (a): Variation of dielectric constant with temperature for pure
and picric acid admixtured TGS crystals at 1 kHz.

125
 Figure 4.23 (b): Variation of dielectric constant with temperature for pure
and picric acid admixtured TGS crystals at 10 kHz.

Figure 4.24(a): Variation of dielectric loss with temperature for pure and
picric acid admixtured TGS crystals at 1 kHz

126
Figure 4.24(b): Variation of dielectric loss with temperature for pure and
picric acid admixtured TGS crystals at 10 kHz

Figure 4.25(a): Variation of AC conductivity with temperature for pure and

127
 picric acid admixtured TGS crystals at 1 kHz

Figure 4.25 (b): Variation of AC conductivity with temperature for pure and
picric acid admixtured TGS crystals at 10 kHz

Figure 4.26(a): Plots of ln σac versus 1000/T for pure and picric acid
admixtured TGS crystals at 1kHz

128
 Figure 4.26(b): Plots of ln σac versus 1000/T for pure and picric acid
admixtured TGS crystals at 10 kHz

Table 4.13: Values of activation energy for pure and picric acid admixtured
TGS crystals
Frequency Values of activation energy (eV)

Pure TGS+10 mole% TGS+20 mole% TGS+30 mole%

TGS
of picric acid of picric acid of picric acid

1 kHz 0.701 0.578 0.412 0.346

10 kHz 1.06 0.812 0.649 0.464

4.3.11 Thermal analysis

TG/DTA studies for picric acid admixtured TGS crystals were carried out

simultaneously and the recorded thermal curves are shown in figures 4.27 to and

4.29.The sharp endothermic peaks show the good degree of crystallinity of the

samples. The values of decomposition point of picric acid admixtured TGS crystals

are given in Table 4.14. As the decomposition point values for picric acid added

129
TGS crystals are observed to be more than that of pure TGS crystal, the picric acid

admixtured TGS samples are found to be harder and this fact is revealed by the

results obtained from hardness studies.

Figure 4.27: TG/DTA thermograms of 10 mole % of picric acid admixtured

TGS crystal

130
Figure 4.28: TG/DTA thermal curves of 20 mole% of picric acid
admixtured TGS crystal

Figure 4.30: TG/DTA thermal curves of 30 mole% of picric acid

admixtured TGS crystal

131
Table 4.14: Values of decomposition point of pure and picric acid admixtured
TGS crystals

Sample Decomposition point

(oC)

Pure TGS 244.27

TGS+10 mole % of picric acid 247.87

TGS+20 mole % of picric acid 248.68

TGS+30 mole % of picric acid 250.42

132
 CHAPTER-V

STUDIES ON VARIOUS PROPERTIES OF PERCHOLORIC

ACID ADMIXTURED TGS SINGLE CRYSTALS
5.1 Introduction
This chapter discussed the growth of percholoric acid admixtured TGS

single crystals by slow evaporation technique. The harvested crystals were

structurally characterized by XRD studies, spectroscopically characterized by

EDAX, FITR, UV-vis-NIR transmittance studies, SHG test, thermally

characterized by TG/DTA analysis, mechanically characterized by Vickers

microhardness test and electrically characterized by dielectric studies.

5.2 Experimental procedure

5.2.1 Material synthesis

Triglycine sulphate (TGS) salt and percholoric acid (HClO4) were taken in 0.9:

0.1 molar ratio and the calculated reactants were dissolved in deionized water, stirred

well, heated at 50 oC to get 10 mole % of percholoric acid admixtured TGS salt.

Similarly, the 20 mole % and 30 mole % of percholoric acid admixtured TGS samples

were prepared.

5.2.2 Solubility studies

The procedure for the measurement of solubility is given in chapter-III. The

solubility curves are depicted in figure 5.1. The solubility of percholoric acid

admixtured TGS sample was estimated for seven different temperatures and the

curve shows a positive solubility gradient in water. It is observed that the solubility

of percholoric acid admixtured TGS samples in water is more compared to that of

pure TGS sample, nitric acid admixtured TGS samples and picric acid admixtured

TGS samples.

133
 Figure 5.1: Solubility curves of pure and percholoric acid admixtured
TGS crystals
5.2.3 Nucleation Kinetics

The procedure for induction period measurements for the samples is given in

the chapter-III. By knowing the values of induction period, the critical nucleation

parameters for percholoric admixtured TGS samples were determined. Results of

induction period (τ) for pure and percholoric acid admixtured TGS crystals are

depicted in the figure 5.2. Induction period decreases as the supersaturation ratio

(S) increases for the samples. When TGS is added with percholoric acid,

induction period decreases and hence nucleation rate increases and this leads to

growth of crystals faster compared to the pure TGS. The procedure for

calculating the nucleation parameters is provided in the chapter-III. From the

figure 5.3, plots of ln  against 1/ (ln S)2 are approximately linear which explains

the classical theory of homogeneous nucleation and the values of slope (m) were

obtained for the samples and the critical nucleation parameters were determined.

The of Gibbs free energy change, radius of critical nucleus, number of molecules

in the critical nucleus and nucleation rate are presented in Table 5.1. It is noticed

134
 the results that the nucleation parameters such as radius of critical nucleus, Gibb’s

free energy change and number of molecules in the critical nucleus decrease with

supersaturation and they increase when concentration of percholoric acid is

increased. For the growth of good quality crystals, the optimized data of solubility

and nucleation are necessary and these data will be useful to understand the type

of nucleation formed in the supersaturated solution.

. Figure 5.2: Variation of induction period  with supersaturation ratio (S) for
solution of pure and percholoric acid admixtured TGS samples

Figure 5.3: The plots of 1/( ln S)2 versus ln τ for solutions of pure TGS
and TGS admixtured with percholoric acid.

135
Table 5.1: Summary of critical nucleation parameters for pure and percholoric
acid admixtured TGS samples.

Sample S Τ ∆G* r* n σ J x1024

( sec) (x10-21) (x10-9) (x10-3) (Nuclei/sec/
(J) m volume)

TGS+10 1.3 6850 11.945 1.214 24 1.14

mole %
HClO4 1.35 5432 9.345 1.1546 18 1.3724
1.4 4612 7.341 1.1234 15 2.2566 1.965
1.45 3321 6.865 0.945 12 2.586
1.5 2510 6.014 0.8764 10 3.678

TGS+20 1.3 6104 15.212 1.3512 31 1.098

mole%
HClO4 1.35 4965 12.387 1.3142 26 1.251
1.4 4321 11.347 1.2521 21 1.856
1.45 2845 10.045 1.1894 17 2.4669 2.312
1.5 1998 8.964 0.9999 13 3.445

TGS+30 1.3 5122 19.321 1.489 36 0.887

mole%
1.35 4652 17.062 1.3942 32 1.123
HClO4
1.4 3321 14.922 1.2847 28 2.5859 1.543
1.45 1900 12.457 1.215 21 1.987
1.5 998 11.125 1.1245 16 2.654

5.2.4 Growth of percholoric admixtured TGS crystals

Single crystals of percholoric acid admixtured TGS were grown by solution

method using slow evaporation solution technique (SEST) at room

temperature(31oC).Saturated solutions of the synthesized salts of percholoric

admixtured TGS salts were prepared separately based on the solubility data. The

solutions were constantly stirred for about 3 hour using a magnetic stirrer and were

filtered using high quality filter papers. Then the filtered solutions were taken in

136
different beakers and covered with porous papers for controlled evaporation.

Transparent, colourless single crystals were harvested within a period of 25-35 days.

The harvested crystals are shown in figures 5.4 to figure 5.6. It is observed that the

morphology of the crystals is altered when the concentration of percholoric acid in

the solutions is increased. This may be due to incorporation of percholoric acid in the

interstitial positions of the host TGS crystals.

Figure 5.4: TGS crystal admixtured with percholoric acid (10 mole %)

Figure 5.5: TGS crystal admixtured with percholoric acid (20 mole %)

137
 Figure 5.6: TGS crystal admixtured with percholoric acid (30 mole %)

5.3 Results and discussions

5.3.1 Structural characterization
Structural characterization was carried out by XRD methods. Both single crystal

and powder XRD methods are used to determine the unit cell parameters. An X-ray

radiation of MoKα (λ= 0.71073 Å) with the help of a single crystal X-ray

diffractometer was used to get the single crystal XRD data for the grown crystals of

percholoric acid admixtured TGS. It is observed that percholoric acid admixtured TGS

crystals crystallize in monoclinic system and the obtained data of unit cell parameters

are listed in Table 5.2. The unit cell parameter values of percholoric acid added TGS

crystals are the same as compared to that of pure TGS crystals. The powder X-ray

diffraction studies using PANalytical model nickel filtered CuK  radiation (λ =

1.54056 Å) were performed to identify the structure and the diffraction planes. All the

reflections of powder XRD patterns of the grown crystals were indexed using the

INDEXING and TREOR software packages. The indexed powder X-ray diffraction

patterns of the grown crystals are depicted in the figures 5.7 to 5.9. From powder XRD

patterns the numerous sharp peaks give a clear cut-proof of the crystalline nature of the

grown samples. Due to incorporation of percholoric acid into TGS samples, some

138
peaks are slightly shifted and a few diffraction peaks are more in the XRD patterns of

percholoric acid added TGS crystals compared to that of pure TGS crystal.

Table 5.2 Unit cell constants of percholoric acid admixtured TGS crystals

S.No. Crystal Lattice parameters Volume of Unit cell

(Å )3

a = 9.308(2) Å
TGS+ 10 mole % of b = 12.342(2) Å
1. c = 5.787(1) Å 654.97
HClO4 α = γ= 90o
β = 107.38o (2)

a = 9.248(1) Å
TGS+ 20 mole % of b = 12.714(4) Å
2. c = 5.912(2) Å 669.84
HClO4 α = γ= 90o
β = 105.50o (2)

a = 9.447(4) Å
TGS+ 30 mole % of b = 12.745(3) Å
3. c = 6.031(1) Å 694.87
HClO4 α = γ= 90o
β = 106.30o (3)

139
 Figure 5.7: Powder XRD pattern of TGS crystals admixtured with
10 mole% of percholoric acid

Figure 5.8: Powder XRD pattern of TGS crystals admixtured with

20 mole % of percholoric acid

140
 Figure 5.9: Powder XRD pattern of TGS crystals admixtured with 30 mole %
of percholoric acid

5.3.2 Confirmation of presence of elements in the grown samples

Energy dispersive analysis by X-ray (EDAX) technique was used to find the

presence of elements in the grown crystals. The EDAX spectra of the samples were

recorded using a computer controlled scanning electron microscope (Model:

HITACHI S-3000H). The recorded EDAX spectra are shown in the figures 5.10 to

5.12. From the spectra, it is confirmed that the elements such as C, O, N, S and Cl

were present in the samples.

141
Figure 5.10: EDAX spectrum of 10 mole % of percholoric admixtured TGS
crystal

Figure 5.11: EDAX spectrum of 20 mole% of percholoric admixtured TGS

crystal

142
Figure 5.12: EDAX spectrum of 30 mole % of percholoric admixtured TGS
crystal

5.3.3 Determination of hardness number, work hardening coefficient, yield

Strength and stiffness constant
Vickers microhardness number (Hv) is calculated using the relation H v =

1.8544 P / d2 kg/ mm2 where P is the load in kilograms, d is the diagonal length of

indentation impression in millimeters (mm). The values of‘d’ are measured using a

Vickers microhardness tester and the values of microhardness number is calculated.

Figure 5.13 shows the variation of Vickers microhardness number (H v) with applied

load for pure and percholoric acid admixtured TGS crystals. For pure and percholoric

acid added TGS samples, the hardness number increases with increase in load

obeying the reverse indentation size effect. The Mayer’s relation P= ad n was used to

determine the work hardening coefficient. The graphs( figures 5.14 (a), 5.14 (b) and

5.14 (c)) are drawn by taking log d versus log P and the values of slope of the

straight lines are equal to the values of work hardening coefficient (n). The

143
calculated values of work hardening coefficient are given in the Table 5.3. According

to Onitsch’s theory, if n is greater than 1.6, the materials are said to be soft materials.

Hence the percholoric acid admixtured TGS crystals belong to the category of soft

materials.

Figure 5.13: Dependence of hardness number (Hv) with loads in grams for
pure TGS and TGS admixtured with percholoric acid

Figure 5.14 (a): Plot of log P versus log d for TGS crystal admixtured
with 10 mole % of percholoric acid

144
Figure 5.14 (b): Plot of log P versus log d for TGS crystal admixtured
with 20 mole % of percholoric acid

Figure 5.14 (c): Plot of log P versus log d for TGS crystal admixtured
with 30 mole % of percholoric acid

145
Table 5.3: Work hardening coefficients (n) of pure TGS and percholoric
acid admixtured TGS crystals

Work
Sample Material type
hardening
coefficient
(n) 2.65
Pure TGS crystal Soft
TGS +10 mole % of 2.53
percholoric acid Soft
TGS +20 mol % of percholoric 2.41
acid Soft
TGS +30 mol % of percholoric 2.32
Soft
acid

The elastic stiffness constant and yield strength for percholoric acid admixtured

TGS crystals are calculated using the relations: Stiffness constant C11=( Hv)7/4 Pascal

and yield strength σy= Hv /3 Pascal. The calculated stiffness constant and yield

strength for different loads for percholoric acid admixtured TGS crystals are tabulated

in Tables 5.4 to 5.6.

Table 5.4: Yield strength and Stiffness constant for TGS crystals
admixtured with 10 mole % of percholoric acid

Sample Load (g) Yield strength (σy) Stiffness constant

x10 6 Pa x 1015 Pa
25 185.75 2.022
TGS + 10 mole % 50 255.19 3.524
of percholoric acid 75 267.37 3.823
100 282.20 4.198

Table 5.5: Yield strength and Stiffness constant for TGS crystals
admixtured with 20 mole % of percholoric acid

Sample Load(grams) Yield Strength Stiffness Constant

(σy) x10 15 Pa
x 10 6 Pa
25 220.34 2.725
TGS + 20 mole % 50 278.77 4.113
of percholoric acid 75 288.96 4.379
100 298.67 4.640

146
Table 5.6: Yield strength and Stiffness constant for TGS crystals admixtured
30mole % of percholoric acid

Sample Load(g) Yield Strength (σy) Stiffness Constant

x10 6 Pa x 10 15 Pa
25 248.65 3.367
TGS + 30 mole % 50 305.27 4.821
of percholoric acid 75 311.80 5.063
100 320.78 5.258

5.3.4 UV-visible spectral studies

The recorded UV-visible transmittance spectra of pure and percholoric acid

admixtured TGS crystals are shown in figure 5.15. This study helps us to find the

suitability of materials in optical device applications. The optical property of the

material gives information regarding the composition, nature and the quality of the

crystal. From the results, it is noticed that the grown crystals have good transmittance

in the visible region. It is observed that the percentage of transmission gets decreasing

with concentration of dopants in the host TGS crystals. Due to the presence of

impurities in the host crystals, the absorption in UV-visible region decreases. A strong

absorption is observed at 236 nm for pure TGS crystal and the strong absorption is

noticed at 241 nm for the percholoric acid added TGS crystals.

Tauc’s plots are drawn to determine the optical band gap for the grown

crystals. The plots of variation of (αℎυ)2 verses ℎυ are known as Tauc’s plots and they

are shown in the figures 5.16, and the band gap energy is calculated by extrapolation

of linear part. The band gap values are tabulated in Table 5.7. Using the transmittance

values, absorbance, extinctioncoefficient, reflectance and refractive index for

percholoric acid admixtured TGS crystals were determined and the used relations are

given in the chapter-III. The variations of extinction coefficient, reflectance,

147
absorptance and refractive index with wavelength are shown in the figures 5.17, 5.18,

5.19 and 5.20.

Figure 5.15: UV-visible transmittance spectra for percholoric acid

admixtured TGS crystals

Table 5.7: Values of cut-off wavelength and band gap for percholoric acid
admixtured TGS crystals

Sample Cut- off wavelength Band gap (eV)

(nm)
Pure TGS crystal 236 5.25

TGS+10 mole % of percholoric 241 5.14

acid
TGS+20 mole% of percholoric 241 5.14
acid
TGS+30 mole% of percholoric 241 5.14
acid

148
. Figure 5.16: Plot of ( αhν)2 versus hυ for pure and percholoric acid admixtured
TGS crystals.

Figure 5.17 : Variation of extinction coefficient with wavelength for pure and
percholoric acid admixtured TGS crystals

149
Figure 5.18: Plot of absorbance versus wavelength for pure and percholoric
acid admixtured TGS crystals

Figure 5.19 : Plot of reflectance versus wavelength for pure and

percholoric acid admixtured TGS crystals

150
Figure 5.20 : Variation of refractive index with wave length for pure and picric
acid admixtured TGS crystals.

5.3.5 Identification of functional groups from FTIR studies

The FTIR spectra were recorded for powdered samples of percholoric acid

admixtured TGS crystals using an FTIR spectrometer by KBr pellet technique in the

range 400 cm-1 - 4000 cm-1 and they are presented in figures 5.21, 5.22 and 5.23.

The various functional groups of the samples have been identified and they are

provided in the Tables 5.8 - 5.10. The assignments for the absorption bands/peaks

of the FTIR spectra are given in accordance with the reported literature [249-251].

For the samples, the broad band covering 3886- 3100 cm -1 indicates the asymmetric

stretching of NH3+ modes. The peak 1650- 1595 cm -1 corresponds NH3+

asymmetric bending. The strong peak region 650 cm-1 – 710 cm-1 can be assigned to

C-Cl stretching vibration .The peak around 550 cm-1 – 620 cm-1 belongs to SO4 2-

scissor bending .

151
Figure 5.21: FTIR spectrum of TGS crystals admixtured with 10 mole % of
percholoric acid

Figure 5.22: FTIR spectrum of TGS crystals admixtured with 20 mole % of

percholoric acid

152
 Figure 5.23: FTIR spectrum of TGS crystals admixtured with 30 mole % of
percholoric acid

Table 5.8: FTIR spectral assignments for TGS crystals admixtured with 10
mole % of percholoric acid

Bands/Peaks (cm-1) Assignments

3425 NH3+ asymmetric and OH stretching

2960 CH2 stretching

2755 NH2 symmetric stretching

2620 Combination band

2480 Absorption due to – CH2 grouping

2095 +
Combination of NH3 asymmetric stretching and
torisonal oscillation

1700 Amide

1620 NH3+ asymmetric bending

1440 CH3 asymmetric bending

1210 OH bending

690 C-Cl stretching vibration

580 SO4 scissor bending

153
Table 5.9: FTIR spectral assignments for TGS crystals admixtured with
20 mole % of percholoric acid

Bands/Peaks (cm-1) Assignments

3920 N-H stretching

3400 NH3+ asymmetric and OH stretching

3000 CH2 stretching

2754 C-H stretching mode

2602 Combination band

2412 Absorption due to – CH2 grouping

2125 Combination of NH3+ asymmetric stretching

and torisonal oscillation

1701 Amide

1625 NH3+ asymmetric bending

1480 CH3 asymmetric bending

1200 OH bending

1120 NH3+ rocking

706 C-Cl stretching vibration

576 SO4 Scissor bending

502 Combination of NH3+ asymmetric stretching

and torisonal oscillation

154
Table 5.10: FTIR spectral assignments for TGS crystals admixtured with 30
mole % of percholoric acid

Bands/Peaks (cm-1) Assignments

3886 N-H stretching

3495 NH3+ asymmetric and OH stretching

NH3+ asymmetric stretching, OH stretch of

3336 water

3173 NH3+ asymmetric stretching

2970 CH2 stretching

2733 C-H stretching mode

2619 Combination band

2353 Stretching of CH3 vibration

Combination band of NH3+ degenerate mode

2053 and NH3+ torsion

1665 Amide

1594 NH3 + asymmetric bending

1394 C–C stretching

1170 OH bending

1097 NH3+ rocking

936 C-C-N stretching

656 C-Cl stretching vibration

548 SO4 Scissor bending

155
5.3.6 Measurement of Second Harmonic Generation (SHG) efficiency

A high intensity was used to comport Kurtz-Perry test. The SHG was

confirmed by the emission of green radiation (532 nm) which was detected and the

values of relative SHG efficiency for 10% of percholoric acid admixtured TGS

crystal, 20% of percholoric acid admixtured TGS crystal and 30% of percholoric

acid admixtured TGS crystal are found to be 0.60, 0.71 and 0.85 respectively and

hence the percholoric acid TGS crystals could be used as the second harmonic

generators. Here it is to be mentioned that pure TGS crystal has no SHG efficiency.

From the results, it is found that SHG efficiency increases with increase in

concentration percholoric acid in the host TGS crystals.

5.3.7 Dielectric characterization

The dielectric characterization of the crystalline samples was carried out using

a precision LCR meter in the frequency range 1 kHz and 10 kHz. The dielectric

constant and loss factor were measured for percholoric acid admixtured TGS crystals

and AC conductivity was calculated. Using the values of AC conductivity, activation

energy was determined. The variations of dielectric constant (εr) with temperature

are presented in the figures 5.24 (a) and 5.24 (b). The temperature dependence of

dielectric loss (tan δ) obtained for the frequency 1 kHz is presented in figure 5.25 (a)

and for 10 kHz in figure 5.25 (b). From the graphs, it is observed that as the

temperature increases, the values of dielectric constant and dielectric loss increase

for percholoric acid admixtured TGS crystals.

156
Figure 5.24 (a): Variation of dielectric constant with temperature for
percholoric acid admixtured TGS crystals at 1 kHz.

Figure 5.24 (b): Variation of dielectric constant with temperature for

percholoric acid admixtured TGS crystals at 10 kHz.

157
Figure 5.25 (a): Variation of dielectric loss with temperature for
percholoric acid admixtured TGS crystals at 1 kHz

Figure 5.25 (b): Variation of dielectric loss with temperature for

percholoric acid admixtured TGS crystals at 10 kHz

158
 The dielectric constant and the dielectric loss values increase with the

increase in temperature for both the frequencies upto the transition temperature and

then they decrease. The percholoric acid admixtured TGS crystal has higher

dielectric constant and loss values compared to that of pure TGS crystal. Increase in

dielectric constant for percholoric acid admixtured TGS crystal at transition

temperature is attributed to free charge carriers created by the dopants. The Curie point

Tc for the samples was not found to be the same. A small shift in Tc is found for

percholoric acid admixtured TGS crystals. Above Tc, the dielectric constant decreases

and obeys Curie-Weiss law. AC conductivity (σac) of the grown crystals for different

frequencies can be determined using the relation σac = 2Пf εr ε0 tan δ where f is the

frequency of AC supply, εr is the dielectric constant, εo is the permittivity of free

space, tan δ is the dielectric loss. AC conductivity values obtained for the grown

crystals for 1 kHz frequency is provided in figure 5.26 (a) and for 10 kHz frequency in

figure 5.26 (b).

The AC conductivity curve shows that the Curie point (T c) is observed to be 51 oC.

The AC conductivity is small at low temperature, which increases with temperature

up to the Curie point. When the temperature of the crystal is increased, there is a

possibility of weakening of hydrogen bond. This results in an enhanced conductivity

in these materials. Figures 5 .27 ( a) and 5 .27(b) show the variation of ln (σac)

with 1000/T for 1 kHz and 10 kHz frequency of pure and percholoric acid

admixtured TGS crystals. From the graphs, activation energy values for the grown

pure and percholoric acid admixtured TGS crystals were determined .

159
 Figure 5.26 (a): Variation of AC conductivity with temperature for pure
and percholoric acid admixtured TGS crystals at 1 kHz

Figure 5.26 (b): Variation of AC conductivity with temperature for pure and
percholoric acid admixtured TGS crystals at 10 kHz

160
Figure 5.27 (a): Plots of ln σac versus 1000/T for pure and percholoric acid
admixtured TGS crystals at 1kHz

Figure 5.27 (b): Plots of ln σac versus 1000/T for pure and percholoric acid
admixtured TGS crystals at 10 kHz

161
5.3.8 Ultrasonic studies

As given in the chapter-III, the same procedure was followed to find the

acoustical parameters for percholoric acid added TGS samples. Table 5.11 provides

the summary of ultrasonic velocity, viscosity and density for the saturated solutions of

percholoric acid admixtured TGS samples. From these studies, it is observed that the

ultrasonic velocity, viscosity and density of the saturated solutions of percholoric acid

admixtured TGS samples are more when compared to the pure saturated solution of

TGS, referring the chapter-III. Table 5.12 gives the summary of other acoustical

parameters for saturated solutions of percholoric acid admixtured TGS samples.

Table 5.11: Data of ultrasonic velocity, viscosity and density for

percholoric acid admixtured TGS supersaturated solutions

Ultrasonic Density ( ρ) Viscosity ( η)

Sample Velocity (V) kg m-3 (10-3)
m/s Nsm-2
2.614
TGS + 10 mole % of 1621 1670
Percholoric acid
TGS+ 20 mole % of 7.357
1634 1683
Percholoric acid
TGS+30 mole % of
Percholoric acid 1642 1695 11.123

162
Table 5.12: Data of the acoustical parameters for percholoric acid
admixtured TGS saturated solutions

βa Lf Пi τ Za

Sample
10-10 10-10 106 (Pascal 10 -12 106

second)
m2N-1 m (s) (kgm-2S-2)

TGS + 10 2.2788 0.3009 69.5045 0.7942 2.7070

mole % of
percholoric
acid

TGS+ 20 2.2254 0.2974 116.640 2.1829 2.7500

mole % of
percholoric
acid

TGS+30 mole 2.1881 0.2949 143.626 3.2452 2.7831

% of
percholoric .

acid

5.3.9 Thermal characterization

The grown crystals of percholoric acid admixtured TGS were thermally

characterized by TG/DTA analysis. TG/DTA thermograms of the samples are

presented in the figures 5.28- 5.30. It is observed that the FTIR spectra of percholoric

acid admixtured TGS crystals are similar except that the change of decomposition

points. The values of decomposition point of percholoric acid admixtured TGS

crystals are noticed to be 256.54 oC , 262.34 oC and 272.20 oC for TGS crystals

163
admixtured with 10 mole % percholoric acid , 20 mole % percholoric acid and 30

mole % percholoric acid respectively .The decomposition point is observed to be

increasing with increasing the concentration of percholoric acid in the TGS crystals.

Figure 5.28: TG/DTA thermo grams of 10 mole % of percholoric acid admixtured

TGS crystal

164
Figure 5.29: TG/DTA thermo grams of 20 mole% of percholoric acid admixtured TGS
crystal

Figure 5.30: TG/DTA thermograms of 20 mole% of percholoric acid

admixtured TGS crystal

165
 CHAPTER-VI

STRUCTURAL, SPECTRAL, MECHANICAL AND ELECTRICAL

STUDIES OF TGS CRYSTALS ADMIXTURED WITH SALICYLIC
ACID
6 .1 Introduction
Salicylic acid is an organic nonlinear optical material and it is used as the

admixtured material to improve the various properties of TGS crystals. The

growth of salicylic acid admixtured TGS single crystals was carried out by slow

evaporation technique. The grown crystals have been characterized by various

studies and the results are presented and discussed.

6.2 Synthesis, solubility, nucleation kinetics and growth

To synthesize 10 mole % of salicylic acid added TGs sample, glycine, sulphuric

acid and salicylic acid were taken in 3: 0.9: 0.1 molar ratio. The reactants were

dissolved in deionized water, stirred well, heated at 50 oC to get the 10 mole% of

salicylic acid admixtured TGS salt. Similar procedure was followed to prepare 20

mole % and 30 mole % of salicylic acid admixtured TGS samples. Solubility of the

salicylic acid admixtured TGS samples with various temperatures was measured by

gravimetrical method. The solubility curves for the samples are presented in the

figure 6.1. From the graph, it is observed that the solubility of salicylic acid

admixtured TGS sample in water increases as the temperature increases and hence

the samples have positive temperature coefficient of solubility. The data of solubility

are used to prepare the saturated solution at a particular temperature. Using the

solubility curves, the supersaturated solutions of salicylic acid added TGS salts were

prepared at the selected supersaturation ratios viz. 1.3, 1.35, 1.4, 1.45 and 1.5. The

induction period was measured for each supersaturation ratio by isothermal method.

The variations of induction period with supersaturation ratio for the samples are

166
presented in the figure 6.2. Using the values of induction period, the critical

nucleation parameters were determined as the procedure given in the chapter-III.

From the figure 6.3, plots of ln  against 1/( ln S)2 are approximately linear which

explains the classical theory of homogeneous nucleation and the values of slope (m)

were obtained. The calculated values of Gibbs’ free energy change, radius of critical

nucleus, number of molecules in the critical nucleus with the supersaturation ratio (S)

and nucleation rate are tabulated in Table 6.1. Studies on nucleation kinetics of

crystalline samples were carried out in order to have the controlled nucleation rate.

The number of crystals produced in the supersaturated solution is expressed as

nucleation rate i.e. the number of crystals produced per unit volume per unit time.

The variables that affect the nucleation rate are pH, supersaturation, temperature and

interfacial tension of the solution. Decrease in induction period and increase in

interfacial tension are expected to increase the nucleation rate. With the optimized

values of induction period, the growth of pure and salicylic acid admixtured TGS

crystals have been grown from aqueous solutions. Growth of salicylic acid

admixtured TGS crystals was carried out by solution method with slow evaporation

technique at room temperature (31oC). Synthesized and re-crystallied salt of salicylic

acid admixtured TGS were used to prepare the saturated solutions and the growth was

carried out by slow evaporation technique. The optimized growth parameters from

the studies of nucleation kinetics were used to grow good quality bulk single crystals.

It took about 30 to 40 days to harvest the crystals. Grown crystals are shown in

figures 6.4 to 6.6 and the grown crystals are found to be non-hygroscopic, transparent

and colorless. It is to be mentioned here that when TGS crystals are added with 30

mole% of salicylic acid, the transparency is less.

167
Figure 6.1: Solubility curves of salicylic acid admixtured TGS
crystals

Figure 6.2: Variation of induction period () with supersaturation

ratio (S) for solution of salicylic acid admixtured TGS.

168
 Figure 6.3: The plots of 1/( ln S)2 versus ln τ for solutions of TGS
admixtured with salicylic acid.

Table 6.1: Summary of critical nucleation parameters for salicylic

acid admixtured TGS samples

Sample S τ ∆G* r* n σ (10-3) J x1024

( sec) (10-21) (10-9) J/m2 (Nuclei/Sec/
(J) m Volume
TGS+10 1.3 6418 10.233 1.1014 17 0.7452
mole % 1.35 5247 8.143 0.9452 11 1.341
of 1.4 4612 6.021 0.8667 8 1.915 1.845
salicylic 1.45 3129 5.012 0.7874 6 2.542
acid 1.5 2041 3.945 0.7225 5 3.541

TGS+20 1.3 5615 11.145 1.214 19 0.312

mole % 1.35 4324 10.045 1.124 13 0.6413
of 1.4 3404 6.945 0.9412 11 2.0669 1.147
salicylic 1.45 2317 5.9652 0.8419 8 1.9412
acid 1.5 1649 4.784 0.7412 6 2.387

TGS+30 1.3 4984 12.885 1.245 22 0.1452

mole % 1.35 3776 11.165 1.1347 15 0.3871
of 1.4 2582 8.784 1.022 13 2.158 0.984
salicylic 1.45 1406 7.158 0.9142 10 1.4212
acid 1.5 964 5.412 0.8115 8 1.6874

169
Figure 6.4: TGS crystal admixtured with salicylic acid (10 mole %)

Figure 6.5 : TGS crystal admixtured with salicylic acid (20 mole %)

Figure 6.6: TGS crystal admixtured with salicylic acid (30 mole %)

170
6.3 Structural analysis

The grown salicylic acid admixtured TGS crystals were characterized

structurally by single crystal XRD studies. The obtained data from single crystal

XRD studies are given in the Table 6.2. From the data, it is noticed that the grown

crystals of salicylic acid admixtured TGS crystals crystallize in monoclinic

structure. The space group and number of molecules per unit cell are found to be

P21 and 2 respectively.

Table 6.2
Unit cell parameters of salicylic acid admixtured TGS crystals

S. No. Sample Cell parameters Volume of unit cell

(Å )3

a = 9.346(2) Å
TGS+ 10 mole % of b = 12.246(1) Å
1. c = 5.937(2) Å 652.44
salicylic acid α = γ= 90o
β = 106.22 o (3)

a = 9.147(1) Å
TGS+ 20 mole % of b = 12.742 3) Å
2. c = 6.027(2) Å 677.65
salicylic acid α = γ= 90o
β = 105.27o (2)

a = 9.728 (3) Å
TGS+ 30 mole % of b = 12.941(2) Å
3. c = 5.828(1) Å 700.76
salicylic acid α = γ= 90o
β = 107.23o (3)

171
6.4 Mechanical studies

Microhardness of crystalline materials is influenced by defect aggregates and

point defects which hinder the motion of dislocations. Vickers microhardness

indentations were carried out on the polished face of the grown crystal with the load

ranging from 25 g to 100 g using Leitz Pyramidal hardness tester fitted with a

diamond pyramidal indenter. Time of indentation was kept as 5 seconds for the

samples. The variations of microhardness number with different loads applied to the

samples are provided in the figure 6.7 and it is noticed that Vickers hardness number

(Hv) increases with the applied load. Hardness when TGS crystals are added with

salicylic acid. The addition of salicylic acid into TGS crystalline samples most

probably enhances the strength of bonding in the host material and hence hardness

number increases. The yield strength and stiffness constant were calculated for the

grown crystals as per the procedure given in the chapter-III. Variations of yield

strength and stiffness constant with the applied load are presented in figures 6.8 and

6.9. It is observed that the mechanical parameters such as yield strength and stiffness

constant increase with increase in the load and these parameters are found to be

increasing when TGS crystals are admixtured with salicylic acid. This may be due to

strengthening of bonds when salicylic acid is in the interstitials of the host TGS

crystals [252-254].

172
Figure 6.7 : Variation of hardness number (Hv) with loads for
salicylic acid admixtured TGS crystals

Figure 6.8 : Variation of yield strength with loads for

TGS crystals admixtured with salicylic acid

173
 Figure 6.9: Variation of stiffness constant with loads for
TGS crystals admixtured with salicylic acid

6.5 Optical studies

The optical transmittance spectra of salicylic acid added TGS crystals are shown

in the figure 6.10. It is observed that the transmittance decreases when TGS crystals

are admixtured with salicylic acid. At about 236 nm, a sharp fall in the transmittance

is observed for all the crystals, which corresponds to fundamental absorption

edge that is essential in connection with the theory of electronic structure. It is

observed that the magnitude of energy band gap for pure and salicylic acid

admixtured TGS crystals are the same. Using the formula Eg = 1240 / λ (nm), the

energy band gap is calculated to be 5.254 eV. The values of extinction coefficient,

reflectance and refractive index for salicylic acid admixtured TGS crystals were

calculated as the procedure given in the chapter- III and the variations of extinction

coefficient, reflectance, and refractive index with wavelength are shown in the figures

6.11, 6.12, and 6.13.

174
Figure 6.10: UV-visible transmittance spectra for salicylic acid
admixtured TGS crystals

Figure 6.11 : Variation of extinction coefficient with wavelength for

salicylic acid admixtured TGS crystals

175
Figure 6.12 : Plot of reflectance versus wavelength for salicylic acid admixtured
TGS crystals

Figure 6.13 : Variation of refractive index with wave length for salicylic
acid admixtured TGS crystals

176
6.6 Identification of elements in the samples by EDAX method

Energy Dispersive X-ray spectroscopy (EDAX) is the technique used to find

the presence of various elements in the pure and doped crystals. The EDAX spectra of

10, 20 and 30 mole % of salicylic acid-admixtured TGS crystals are recorded using a

computer controlled Scanning Electron Microscope (Model: HITACHI S-3000H).

The recorded EDAX spectra are shown in figures 6.14- 6.16. From these, it is

confirmed that the elements such as C, O, N and S were present in the samples.

Figure 6.14: EDAX spectrum of 10 mol% of Salicylic acid admixtured TGS

crystal

177
Figure 6.15: EDAX spectrum of 20 mole % of salicylic acid admixtured TGS
crystal

Figure 6.16: EDAX spectrum of 30 mol% of salicylic acid admixtured TGS

crystal

178
6.7 Kurtz-Perry technique

Kurtz and Perry technique was used to check the NLO property of the samples.

The grown crystals of salicylic acid added TGS were powdered and a high intensity

Nd:YAG laser (λ=1064 nm) was used as source. The SHG was confirmed by the

emission of green radiation (532 nm). The values of relative SHG efficiency of

salicylic acid admixtured TGS crystals were found with reference to KDP. The

obtained values of SHG efficiency are 0.86, 1.11 and 1.23 for TGS crystals

admixtured with 10 mole %, 20 mole % and 30 mole % respectively .Compared to

the values of SHG efficiency of picric acid and percholoric acid added TGS

crystals, the salicylic acid added TGS crystals have more SHG efficiency and hence

these crystals are the potential candidates for NLO applications.

.
6.8 Spectral studies by FTIR technique

FTIR spectral studies are important in the investigation of molecular structure,

examination of stretching, bending, twisting and vibrational modes of atoms in a

molecule and hence to identify the functional groups of samples. The FTIR spectra

were recorded for powdered samples of salicylic acid admixtured TGS using an

FTIR spectrometer by KBr pellet technique. The recorded FTIR spectra of salicylic

acid admixtured TGS crystals are shown in figures 6.17, 6.18 and 6.19. The

observed vibrational wave numbers and their assignments for salicylic acid

admixtured TGS crystals are tabulated in Tables 6.3 - 6.5. The IR spectral

assignments for the bands/ peaks of FTIR spectra of the samples are given in

accordance with the reported in the literature [188,255]. The broad band covering

3912 cm-1- 3400 cm-1 region indicates the stretching frequencies of superimposed O-

H and NH3+ modes. Multiple combination and overtone bands of CH 2 have been

179
observed in the region 3002 cm-1 2985 cm-1. The absorption region 1700 cm-1 – 1650

cm-1 is assigned to C= O stretching of COOH group. Asymmetric bending vibrations

and the absorption bands occurs at 500 cm-1 are due to symmetric NH2 bending

vibrations. The peaks around 700 cm-1 to 650 cm-1 assigned as SO42- stretching

vibrations lie in the same envelope of C-N stretch. The broadening or narrowing of

some of the absorption bands / peaks of the FTIR patterns of the admixtured TGS

crystals indicates that salicylic acid have entered into the host TGS crystals.

Figure 6.17: FTIR spectrum of TGS crystals admixtured with

10 mole % of salicylic acid

180
Figure 6.18: FTIR spectrum of TGS crystals admixtured with 20 mole % of
salicylic acid

Figure 6.19: FTIR spectrum of TGS crystals admixtured with 30 mole % of

salicylic acid

181
Table 6.3: FTIR spectral assignments for TGS crystals admixtured with
10 mole % of salicylic acid

Bands/Peaks (cm-1) Assignments

3785 N-H stretching

3385 NH3+ asymmetric and OH stretching

3002 CH2 stretching

2812 C-H stretching mode

2360 Stretching of CH3 vibration

2080 Combination of NH3+ asymmetric stretching

and torisonal oscillation

1985 NH3+ asymmetric bending

1650 Amide

1425 CH3 asymmetric bending

1050 OH bending

935 C-C stretching

705 SO4 Scissor bending

450 NH3+ torsion

182
Table 6.4: FTIR spectral assignments for TGS crystals admixtured with
20 mole % of Salicylic acid

Bands/Peaks (cm-1) Assignments

3395 NH3+ asymmetric and OH stretching

2976 CH2 stretching

2697 C-H stretching mode

2361 Stretching of CH3 vibration

2135 Combination of NH3+ asymmetric stretching

and torisonal oscillation

1924 NH3+ asymmetric bending

1651 Amide

1386 C–C stretching

1051 OH bending

674 SO4 Scissor bending

439 NH3+ torsion

183
Table 6.5: FTIR spectral assignments for TGS crystals admixtured with
30 mole % of salicylic acid

Bands/Peaks (cm-1) Assignments

3912 N-H stretching

3412 NH3+ asymmetric and OH stretching

3210 NH3+asymmetric stretching, OH stretch of

water

2984 CH2 stretching

2720 C-H stretching mode

2380 Stretching of CH3 vibration

2153 Combination of NH3+ asymmetric stretching

and torisonal oscillation

1962 NH3+asymmetric bending

1692 Amide

1410 CH3 asymmetric bending

1075 OH bending

901 C-C stretching

690 SO4 Scissor bending

448 NH3+ torsion

6.8 Dielectric behavior of the samples

184
 An LCR meter (Agilent 4284A) in the frequency range 1 kHz and 10 kHz

was used to measure the capacitance of the sample and hence dielectric constant

was calculated using this relation εr = Cs/Ca where Ca is the capacity of the

condenser without sample and Cs is the capacity of the condenser with sample. The

variation of dielectric constant and dielectric loss with temperature are displayed

in f igures 6.20(a), 6.20(b), 6.21(a) and 6.21 (b). It is observed that the values of

dielectric constant and loss are found to be decreasing with increase in frequency.

When temperature is increased, the dielectric constant and loss factor increases upto a

temperature known as Curie temperature and then they decrease with increase in

temperature. This indicates that the grown crystals are ferroelectric samples. AC

conductivity is calculated for salicylic acid admixtured TGS crystals at different

temperatures for the frequency of 1 kHz and 10 kHz and the variations are shown in

figures 6.22 (a) and 6.22 (b). It can been observed that, AC conductivity increases

with increase in impurity concentration of the admixture. Addition of admixtured

impurity ( salicylic acid) into the lattice of TGS crystals increases the charged defects

and this leads to increase in conductivity[257- 259].

185
Figure 6.20 (a): Variation of dielectric constant with temperature for
salicylic acid admixtured TGS crystals at 1 kHz

Figure 6.20 (b): Variation of dielectric constant with temperature for

salicylic acid admixtured TGS crystals at 10 kHz

186
Figure 6.21 (a): Variation of dielectric loss with temperature for
salicylic acid admixtured TGS crystals at 1 kHz

Figure 6.21 (b): Variation of dielectric loss with temperature for

salicylic acid admixtured TGS crystals at 10 kHz

187
 Figure 6.22 (a): Variation of AC conductivity with temperature for
salicylic acid admixtured TGS crystals at 1kHz

Figure 6.22 (b): Variation of AC conductivity with temperature for

salicylic acid admixtured TGS crystals at 10 kHz

188
6.9 TG/DTA analysis

The t hermogravimetric and D ifferential T hermal analys is are the

familiar techniques [260] to find the thermal stability and to identify the various

transitions (exothermic and endothermic) of a substance. The obtained TG/DTA

patterns for the salicylic acid admixtured TGS samples are shown in figures 6.23 to

6.25. The major weight loss between the temperature range 210oC to 260 o
C is

observed for the samples. The values of decomposition point for 10 mole % of

salicylic acid admixtured TGS crystal, 20 mole % of salicylic acid admixtured TGS

crystal and 30 mole % of salicylic acid admixtured TGS crystal are found to be

247.33oC, 251.78oC and 253.19o C respectively and hence the salicylic acid

admixtured TGS crystals have thermally stable upto 253.19oC.

Figure 6.23: TG/DTA thermograms of 10 mole % of salicylic acid admixtured

TGS crystal

189
Figure 6.24 : TG/DTA thermograms of 20 mole % of salicylic acid admixtured
TGS crystal

Figure 6.25: TG/DTA thermograms of 30 mole % of salicylic acid admixtured

TGS crystal

190
 CHAPTER-VII
SUMMARY, CONCLUSIONS AND SUGGESTIONS
FOR FUTURE WORK

7.1 Introduction

In view of technological importance, crystal growth and characterization of

ferroelectric and nonlinear optical materials have a always been an interesting

research among scientists and researchers for many years. In optics is concerned,

crystals can be classified into two categories viz. linear optical and nonlinear optical

crystals. In a linear optical crystal, polarization varies linearly with applied electric

field whereas in a nonlinear optical crystal, polarization varies nonlinearly with

applied electric field. Ferroelectric crystals are the nonlinear crystals which have

many applications in the manufacture of transducers, high value capacitors, electric

amplifiers, infrared detectors and memory devices etc. Ferroelectric crystals are the

non-centrosymmetric materials and they may exhibit nonlinear optical properties. In

this research work, an important ferroelectric crystal viz. Triglycine Sulphate (TGS)

crystal has been considered for the study. Growth of pure (undoped) TGS crystals and

TGS crystals admixtured with some acids such as nitric acid, picric acid, percholoric

acid and salicylic acid with different concentrations was carried out by solution

method and various properties of the grown crystals were studied.

7.2 Summary and conclusions

Analar Reagent (AR) grade of glycine and concentrated sulphuric acid in the

molar ratio of 3:1 were used for synthesis of pure TGS salt. Some organic and

inorganic acids such as nitric acid, picric acid, percholoric acid and salicylic acid were

used as the admixtures or added impurities to the solution of TGS to obtain

191
admixtured TGS salts. The various concentrations of admixtures added to the

solutions of TGS were 10 mole %, 20 mole % and 30 mole %. In this work, total

number of samples synthesized was 13 and they were i) pure TGS salt, ii) 3 samples

of nitric acid admixtured TGS, iii) 3 samples of picric acid admixtured TGS, iii) 3

samples of percholoric acid admixtured TGS and iv) 3 samples of salicylic acid

admixtured TGS.

Solubility studies in the temperature range 30-60 oC for all synthesized

samples were performed by gravimetrical method. It is found that solubility of the

samples increases with increase in temperature and also increases with increase in the

concentration of admixtures or the added impurities. Based on the theory of

homogeneous nucleation, critical nucleation parameters such as Gibb’s free energy

change, radius of critical nucleus, number of molecules in the critical nucleus and

interfacial tension were determined using the measured values of induction period for

the synthesized samples. In the present work, induction period measurements were

performed for the selected supersaturation ratios such as 1.3, 1.35, 1.4, 1.45 and 1.5

for all the samples. It is observed that induction period decreases as supersaturation

ratio increases and it is found that the nucleation parameters (except nucleation rate)

increase when TGS samples are added with nitric acid, percholoric acid, picric acid

and salicylic acid.

Single crystals of undoped and admixtured TGS samples were carried by

solution method with slow evaporation technique. In accordance with the solubility

data, saturated solutions of synthesized salts of undoped and admixtured TGS were

prepared and the solutions were constantly stirred for about 2 hours using a magnetic

stirrer and were filtered using Whatmann filter papers. Then the filtered solutions

were kept in borosil beakers covered with porous papers. To maintain the growth

192
temperature (31oC) constant, a constant temperature bath was used. The allowed

growth period was about 25-35 days. All the synthesized samples were grown in the

form of big sized single crystals by seeded solution method. The grown crystals have

been subjected to various studies like single crystal XRD and powder XRD studies,

FTIR studies, dielectric studies, dielectric studies, TG/DTA studies, measurement of

density, UV-visible transmittance studies, determination of reflectance, extinction

coefficient, refractive index and optical band gap, EDAX studies, SHG studies,

microhardness studies and ultrasonic studies. The effect of incorporation of

impurities such as nitric acid, picric acid, percholoric acid and salicylic acid on the

various properties of TGS crystals has been investigated.

The floatation method was employed for the precise determination of density.

From the measurements of density it is concluded that density increases with

increase in concentration of impurities. The presence of impurities was estimated

by recording EDAX spectra. From single crystal and powder XRD studies, the

crystal structure and diffracting planes of the grown crystals have been identified.

All the grown crystals of this work were found to crystallize in monoclinic

structure. It is observed that there is no systematic variation in the values of cell

parameters (except cell volume) when TGS crystals are admixtured nitric acid,

picric acid, percholoric acid and salicylic acid. Various functional groups of the

grown crystals of this work were identified from FTIR spectra. UV-visible

transmittance spectra of pure and admixtured TGS crystals were recorded in the

wavelength range 190 -1100 nm and the linear optical constants such as band gap,

reflectance, extinction coefficient, absorbance, refractive index were determined.

Using the UV-visible spectra, Tauc’s plots were drawn to find the optical band gap.

The grown crystals such as undoped TGS crystal, nitric acid admixtured TGS

193
crystals and salicylic acid admixtured TGS crystals have the same cut-off

wavelength at 236 nm. From this study, it is observed that the percentage of

transmission decreases in the visible region as the concentration of impurities

increases and slight shift of cut-off wavelength is noticed in the case of percholoric

acid and picric acid admixtured TGS crystals. Second Harmonic Generation (SHG)

studies for all the samples have been carried out and it is observed that pure TGS

and nitric acid admixtured TGS crystals have no SHG efficiency, but percholoric

acid, picric acid and salicylic acid admixtured TGS crystals have shown second

harmonic generation. It is observed that salicylic acid admixtured TGS crystals

have more SHG efficiency than that of other samples.

Pure TGS single crystals and all the admixtured TGS crystals were

subjected to Vickers microhardness test with the applied load varying from 25 g to

100 g to understand the mechanical property. Microhardness measurements have

proved that all the crystals obey reverse indentation size effect. In the

present investigation the work hardening coefficient of the sample crystals are

found to be greater than 2 and hence it was concluded that the grown crystals

belong to the category of soft materials. The thermal stability of the samples was

tested by TG/DTA studies. Comparison of the TG/DTA thermal curves, it is

observed that the decomposition point is altered when TGS crystals are admixtured

with nitric acid, picric acid, percholoric acid and salicylic acid. Due to high

hardness, good thermal stability, NLO property and good transparency. The grown

admixtured TGS crystals may be better alternative candidates than pure TGS

crystals in technological, industrial and academic uses.

194
 Ultrasonic studies for the samples have been carried out at constant frequency

of 2MHz. Ultrasonic velocity, density, compressibility and other acoustical

parameters were determined from ultrasonic studies. It is noticed that the values of

ultrasonic velocity, density and viscosity and other acoustical parameters change

when TGS solutions are added with admixtures. Dielectric studies for the samples

have been carried out at various frequencies and at different temperatures. Using

the data of dielectric constant and loss factor, AC electrical conductivity values and

activation energy values were determined. From the results, it is observed that the

electrical properties of grown crystals have been modified when the TGS crystals

are admixtured with nitric acid, picric acid, percholoric acid and salicylic acid. The

low value of dielectric constant and loss factor of the grown crystals indicate that

there will be minimum losses and hence these crystals may be more useful for high

speed electro-optic modulation, ferroelectric and other nonlinear optical

applications.

7.3 Future scope

With an interest to discover new materials, in the present investigation, TGS

single crystals admixtured with inorganic and organic acids (nitric acid, picric acid,

percholoric acid and salicylic acid) with 10 mole%, 20 mole% and 30 mole%

percentage of concentration have been grown and studied. The method used for the

growth of pure, and the admixtured TGS crystals is the solution method with slow

evaporation technique. In the future, it is proposed to grow big-sized and

unidirectional doped ferroelectric crystals for device applications by slow cooling

and Sankaranarayanan-Ramasamy (SR) methods. In this work, only second order

nonlinear optical (NLO) properties were studied for the samples. Z-scan technique

can be used to study the third-order NLO properties in the future. The pyroelectric,

195
hysteresis and piezoelectric studies of the samples can also be carried out. It has

been proposed to fabricate sensors using single crystals of this work and crystalline

perfection can be carried out by HRXRD analysis. Laser damage threshold studies

can be carried out for samples. Dislocation, surface defects and morphology can be

characterized by chemical etching followed by etch pit examination using optical

microscopy. Attempts can also be made to grow mixed crystals to improve the

optical nonlinearity and the sophisticated techniques such as AFM, SEM etc could

be used to characterize the grown samples. Fabrication of ferroelectric and

nonlinear optical devices could be carried out using the grown crystals of this work

in the future.

196
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