CHAPTER 1

STRUCTURE OF SOLID MATERIALS

CONTENTS

• Crystal structure
– space lattice,
– Miller indices,
– lattice planes,
– hexagonal closed packing (hcp) structure,
– characteristics of an hcp cell.
– Imperfections in crystal: Point defects (Frenkel
and Schottky).
• X – ray diffraction
– Bragg’s law and Bragg’s spectrometer,
– powder method,
– rotating crystal method.
INTRODUCTION
Three States of Matter:
– Solids
– Liquids
– Gases
Solids:
The aggregates of atoms which preserve their
volumes and shapes unless subjected to large
external force are called solids”.
There are two types of solids :
Amorphous (non-crystalline) and
Crystalline
These are shown in Fig.
 Difference Between Amorphous and Crystalline Solids
Amorphous Crystalline

• Amorphous solids (means • A crystalline solid is the

without form) are the one in which there is a
solids which lacks the regular repeating pattern
regular arrangement of in the structure, or in
atoms or molecules and other words, there is long-
hence they have a short range order :
range order or no order in ABCABCABCABC…
their structure : • Have sharp melting point
ABCBBACBCACCB... (because all bond are
• Do not have sharp melting equally strong)
point (because all bonds • Anisotropic (Physical
are not equally strong) properties are different
• Isotropic (Physical in different directions)
properties are same in • Examples: diamond, table
different directions) salt, ice, methanol, sodium
• Examples: glass, wax, chloride, etc.
plastics, etc.
CRYSTAL LATTICES

A lattice is an infinite, regular array of points in

space.

# In the definition it should be noticed that no mention

is made of atoms or any physical objects, just points
in space - no more, no less. Hence we treat the lattice
as a mathematical abstraction. Therefore, it is clear
that there is no lattice inside the crystal. Even if we
look the crystal through a powerful microscope we
will not be able to see the lattice points, but rather
atoms or groups of atoms. The lattice provides the
'recipe' that determines how the atomic or molecular
units are to be repeated throughout the space to
make the crystal structure.
 Plane Lattice
Consider an array of points in such a way that the
environment about any point is identical with the
environment about any other point. Such an array of
points in two dimensions is shown in Fig. and is called a
plane lattice.

For constructing a two dimensional lattice, choose any two

convenient axis such that the points lie at equal intervals a
and b along these axis as shown in the Fig. There are
generally 5 lattices in two dimensions: Oblique, Square,
Hexagonal, Rectangular and Centered Rectangular lattice.
Space Lattice
If this array of points is extended to three
dimensions then the array of points is called space
lattice. For constructing the space lattice the points
are arranged at equal intervals c in the third
direction also. There are 14 space lattices in total,
called Bravais Lattice.

Thus a lattice may also be defined as a parallel net

like arrangement of points such that the environment
about any point is identical with the environment
about any other point.
Basis
A basis is defined as an assembly of atoms, ions or
molecules identical in composition, arrangement and
orientation.

Basis consists of the simplest arrangement of atoms

which is repeated at every point in the lattice to build up
the crystal structure.

The number of atoms in a basis may be one as in case of

many metals and inert gases, but could be as large as
1000 in many structures.

In ionic crystals, a basis is composed of two distinct

types of ions. For example, Na+ and Cl- in a NaCl crystal.
When basis is attached identically to each lattice point,
the actual crystal structure is formed as shown in the
Fig.

The relation can be written as

Lattice + Basis = Crystal Structure
 UNIT CELL
A unit cell is a region of space which when repeated by
primitive translation vectors fills all space. Thus a unit
cell is defined as the smallest geometrical figure, the
repetitions of which give the actual crystal structure.

The choice of the unit cell is not unique. It can be

constructed in a number of ways, but the unit cell should
be chosen in such a way that it conveys all the symmetry
of a crystal lattice, by having shortest possible size,
which makes the mathematical calculations easy.
Each atom or molecule in a unit cell is considered as a
lattice point. The distance between the two atoms or ions
of the same type is the ‘length of the unit cell’.
Primitive and Non - primitive unit cell

A unit cell which contain just one lattice point is

called primitive unit cell. This cell is the smallest
part of the lattice which when repeated would
reconstruct the entire crystal structure. It is a
minimum volume unit cell and is denoted by the
letter p. A unit cell which contain more than one
lattice point is called non - primitive unit cell.
These two cells are shown in the Fig.
For a three dimensional case, the unit cell is a
parallelopiped formed by basic vectors a, b and c as
concurrent edges and the angles ,  and , between (b,
c), (c, a), and (a, b) respectively as explained in the
following Figures.
Thus, in general, a unit cell may be defined as the smallest
volume of a solid from which the entire crystal may be
constructed by translational repetitions in 3-dimension and
which represent fully all the characteristics of a particular
crystal. In Fig. a three dimensional unit cell is shown by the
shaded portion.
Unit cell of NaCl
Lattice Parameters
In a unit cell the vectors a, b and
c are called translation
vectors or primitive basis
vectors. In two dimensionsn the
area of the unit cell is (a x b)
while in three dimension the
volume of the unit cell is (a x
b).c . In Fig. the direction of
the primitive basis vectors
defines the crystallographic
axis. The angles between these
axis are called interfacial
angles, which are ,  and ,
between (b, c), (c, a), and (a, b)
respectively. Primitive vectors
and interfacial angles together
are called lattice parameters.
CRYSTAL SYSTEMS AND BRAVAIS LATTICES
Crystals of different substances have similar shapes and
hence the crystals are classified into the so called crystal
systems depending upon their axial ratio and the interfacial
angles ,  and . In three-dimension, there are 7 crystal
systems. Bravais showed that throughout the seven crystal
systems there are fourteen unique lattice types possible.
These are known as Bravais or space lattices. These seven
crystal systems with examples are :
• Cubic(CsCl, NaCl, Cu)
• Tetragonal(SnO2)
• Orthorhombic(PbSO4, MgSO4)
• Monoclinic(FeSO4, LiSO4  H2O)
• Triclinic(FeSO4  5H2O, K2Cr2O7)
• Trigonal (Rhombohedral)(Sb, As, CaCO3)
• Hexagonal(Zn, Cd, Ni, As, SiO2)
The characteristics features of these crystal systems and the
corresponding Bravais lattices are as follows:
No. Crystal class Intercepts Angles between Bravais space lattice
on Axes Axes
1 Cubic abc       900 Simple, body-centred,
face-centred
2 Tetragonal abc       900 Simple, body-centred

3 Orthorhombic a  b  c       900 Simple, body-centred,

face-centred,
Base(side)-centred
4 Trigonal abc       900 Simple

5 Hexagonal abc     900, Simple

  1200
6 Monoclinic abc     900   Simple, base-centred
7 Triclinic abc  Simple
MILLER INDICES
The crystal structure may be regarded as made up of an
aggregate of a set of parallel equidistant planes passing
through at least one lattice point or a number of lattice
points. These planes are known as Lattice Planes. For a
given crystal, lattice planes can be chosen in different
ways as shown in Fig.
In order to designate a lattice plane, British mineralogist
William H. Miller, in 1839, developed a method by using
three numbers (h k l) which are known as Miller Indices.
Miller Indices are the three smallest possible integers,
which have the same ratio as the reciprocals of
intercepts of the plane concerned on the three axis.

# Miller indices are integer sets that were created to

distinguish directions and planes in a lattice. They are used
primarily in crystalline structures because they describe
planes in relation to a larger lattice in relative terms, as
opposed to absolute terms. An example of this is describing
planes in a building, Miller indices would distinguish the floor
from the walls, and north wall from west wall, however it
would not distinguish the 4th floor from the 5th floor. This
is useful in crystal lattices because the planes are the same
in many directions(like floors in a tall building).
• Important points:

• Miller indices define the orientation of the plane

within the unit cell

• If a set of planes is perpendicular to any of the axes,

it would cut that axes at , hence the Miller index
along that direction is 1/ = 0.

• If a plane to be indexed has an intercept along the

negative portion of a coordinate axis, a minus sign is
placed over the corresponding index.

• The Miller Index defines a set of planes parallel to

one another (remember the unit cell is a subset of
the “infinite” crystal), e.g., (002) planes are parallel
to (001) planes, and so on.
Let us take an example to find the Miller Indices of a given
plane(see Fig.):

• Intercepts: 2a 1b 1c

• Dividing by unit translation vectors:

2a/a 1b/b 1c/c = 2 1 1

• Taking the reciprocals: ½ 1/1 1/1

• Reducing to whole numbers: 1 2 2

• Miller indices: (122)

In this example, the plane
shown (shaded) cuts the a
length (along the x-axis) at
1/2.

The same plane cuts the b

length (along y) just once at 1
and the c length (along z) at
1/2.

Thus, the Miller indices for

this set of planes would be (2
1 2).
Here the given plane
is perpendicular to y
and cuts at , 1, 

Therefore the Miller

indices for this plane
will be (0 1 0)
For a given Miller indices, a plane can be drawn by taking
the reciprocals of the given indices on the corresponding
axes. Some of the planes and their corresponding Miller
indices are shown in following Figures.
(a) (100) planes, (b) (010) planes,
(c) (111) planes, (d) (110) planes.
INTERPLANER DISTANCE OR SPACING
Interplaner spacing is defined as the perpendicular
distance dhkl between corresponding planes. It is also
perpendicular distance from the origin to the set of parallel
planes(see Fig.)
• To calculate the interplaner
distance, let us take a simple
unit cell in which the coordinate
axes are orthogonal.
• Let us consider set of planes
defined by Miller indices (hkl)
with the reference plane passing
through the origin O.
• The next plane, as shown in Fig.
cuts the intercepts a/h, b/k and
c/l on x, y and z axes
respectively.
• Draw a normal ON = d to the
plane ABC.
• ‘d’ the length of the normal from
the origin is the distance
between the adjacent planes and
is the required interplaner
spacing.
Now, from the Fig. we have
d d d
cos   cos   cos  
a/h b/k c/l
Using the cosine theorem cos 2   cos 2   cos 2   1
2 2 2
 d   d   d 
we have       1
 a/h b/k  c/l 
1

 h2 k 2 l 2  2
solving we get d  2  2  2
a b c 
For a cubic lattice, a = b = c, therefore, we get
a
d hkl 
h2  k 2  l 2
Also, For a cubic lattice,
d100 = a, d110 = a/2 and d111 = a/3.
Physical Parameters for Crystal Structure
(i) Number of Atoms per Unit Cell
Number of atoms per unit cell determines how closely the
solid is packed and is given by
N = Nc/8 + Nf/2 + Ni
here Nc is the number of corner atoms, Nf the number of
face centred atoms and Ni the number of body centred
atoms(see Fig.).

For SC crystal : In a SC crystal, there are 8 atoms only,

each at one corner. Each atom is shared by 8 unit cells.
Therefore, we have
N = Nc/8 = 8/8 = 1
For BCC crystal :N = Nc/8 + Nf/2 + Ni = 8/8 + 0 + 1 = 2
For FCC crystal :N = Nc/8 + Nf/2 + Ni = 8/8 + 6/2 + 0 = 4
(ii) Coordination Number (CN)
In a crystal, the number of nearest neighbours of the
same type and at equal distances from the given atom
is called coordination number.
For SC : The corner atoms are the nearest neighbours
of each other. Here CN = 6(see Fig.) which is a group
of 8 unit cell and atom at the centre has six corner
atoms as its nearest neighbours).
For BCC : In this case all the corner atoms are at
equal distances from the body centered atom.
Hence CN = 8.

For FCC : Here the nearest neighbours of any

corner atom are the face centered atoms of the
surrounding unit cells. Now for any corner atom
there are 4 face centered atoms in each plane
and there are three such planes. Therefore, CN =
12.
(iii) Atomic Radius and Nearest Neighbour Distance (NND)
In a crystal the atoms are assumed to be spheres in contact.
Now atomic radius is defined as half the distance between
the nearest neighbours in a crystal of pure element, i.e.,
the distance between the centres of neighbouring atoms.
For SC : In a SC structure, corner atoms are the nearest
neighbours and are in touch with each other. If the side of
the unit cell is ‘a’ and ‘r’ be the radius , then
2r = a or r = a/2
Now Nearest Neighbour Distance(NND) is given by 2r
Therefore, NND = 2r = a
For FCC : r = 2 a/4

NND = a/2
For BCC : r = 3 a/4
NND = 3 a/2.
(iv) Atomic Packing Fraction (or Factor) (APF)
It is defined as the ratio of the volume of the atoms occupying the unit
cell to the volume of the unit cell. It is also called relative packing
density.
APF = Volume occupied by the atoms in a unit cell / Volume of the
unit cell.
SC Crystal : No. of atoms/unit cell = 1
Volume of one atom = 4/3 r3
Side of the unit cell = a = 2r
Volume of the unit cell = a3
APF = = = /6 = 0.52 = 52%.
BCC Crystal : No. of atoms/unit cell = 2
Volume of two atoms = 2x4/3 r3
Side of the unit cell = a = 4r/3
Volume of the unit cell = a3
APF = = = 3/8 = 0.68 = 68%.
FCC Crystal : No. of atoms/unit cell = 4
Volume of four atoms = 4x4/3 r3
Side of the unit cell = a = 4r/2
Volume of the unit cell = a3
APF = = = 2/6 = 0.74 = 74%.
Hexagonal Close Packed Structure (hcp)

In a closed packed structure the constituent atoms are so

arranged as to occupy minimum possible volume. One of
the most effective arrangement for packing of atoms in
a plane is shown in Fig. Here identical spheres(atoms) are
arranged in a single close packed layer(say ‘A’) with each
sphere touching six other spheres in a plane. This layer
then assumes hexagonal shape. A second layer can now
be added over this by placing spheres in the hollows ‘B’
which are formed by three spheres in the bottom layer.
Now in the third layer the spheres are placed directly
over the spheres of first layer, i.e., ‘A’. In this case the
stacking is AB AB AB …, and the resulting three-
dimensional structure is termed as hexagonal closed
packed (hcp) as shown in Fig.
 Shown displaced for clarity
HCP

+ + =

A B A
HCP

Unit cell of HCP (Rhombic prism)

Note: Atoms are coloured differently but are the same

Characteristics of HCP
Coordination No. = 12
Number of atoms per unit cell = 6
c/a Ratio of HCP:

h = height of the equilateral triangle

a = lattice constant

h a  a 2
  2
2
1
 1  a 
4
3
4
a
Distance x from an atom to the middle of the triangle:

2 2 3 1
x  h   a  a
3 3 4 3
c 1 2
 a x  a  a 
2 2
a 2 2

2 3 3
c 8
  1.633
a 3
DEFECTS IN SOLIDS

No real crystal is perfect. Real crystals feature defects or

irregularities in their ideal arrangements and it is these
defects that critically determine many of the electrical
and mechanical properties of real materials.

Ideally a perfect crystal is the one in which atoms are

arranged in perfectly regular manner in all directions. The
deviations of crystals from their perfect periodicity are
called defects or imperfections.

These imperfections can be of different types such as:

point defects (zero–dimensional defects),

line defects (one–dimensional)
defects over a surface or a plane (two–dimensional) and
volume defects (three–dimensional).
Point Defects
A point defect is a very localized imperfection in the
regularity of a lattice and it does not spread over more
than one or two lattice spacings. These defects are
observed in metallic crystals(vacancies, substitutional
impurity and interstitials) as well as in ionic
crystals(Schottky and Frenkel) and are discussed here in
brief.
Vacancies
The absence of an atom or ion from a normally occupied
site in a crystal is called a vacancy(see Fig.)
Substitutional Impurity
In this kind of defect, a foreign atom occupy a regular
site in the crystal structure(see Fig.), i.e., .substitutional
atom replaces the host atom from its position. For
example, when a pure semiconductor crystal of Silicon or
Germanium is doped with a trivalent or pentavalent
impurity, we call it a substitutional impurity.
Interstitial Impurity
An interstitial is an atom or ion which can be inserted
into the voids between the regularly occupied sites.
In a closed packed arrangement of atoms the packing
fraction is generally less than one. Therefore an
extra atom, of smaller size than the parent atom, can
enter the interstitial space without disturbing the
regularly positioned atoms. Such an extra impure
atom is called interstitial impurity while an extra
atom in an interstitial position is called self –
interstitial atom,as shown in Fig. If the size of the
extra atom is not small then it will produce atomic
distortion.
Schottky Defect
In a metal, a vacancy is created if an atom is missing
from its lattice position. In ionic crystals, a cation – anion
pair will be missing from the respective lattice sites, as
shown in Fig. Creation of such a pair, of one cation
vacancy together with one anion vacancy, is called
Schottky defect. Thus the interior of the ionic crystals
remain electrically neutral.

Frenkel Defect
When an atom or ion leaves its normal position or site and
is found to occupy another position in the interstice we
get a Frenkel defect. Thus, in this case, two
imperfections are created – an interstitial and a vacancy
as shown in Fig. Normally anion leaves its parent site and
occupy the interstitial space. These defects are
dominant in open structures such as silver halides. Also a
Frenkel defect does not affect the electrical neutrality
of a crystal.
 Cation

Anion

Interstitial ion

Missing anion site Missing cation site Missing cation site

(a) Shottkey defect (b) Frenkel defect

Concentration of Schottky Defects
Consider a pure crystal composed of equal numbers of
positively and negatively charged ions. Let us assume that
it contains N ions and n Schottky defects, i.e., n – cation
vacancies and n – anion vacancies.

Now, for a crystal, Helmholtz free energy F is defined as

F = U – T.S …(1)
here U is the internal energy, S is the entropy(the measure
of disorder in a system) and T is the temperature.

Let E is energy required to produce a pair of vacancies in the

interior of a crystal. Then the increase in internal energy,
when n – vacancies are formed, is
U = nE …(2)
As the number of defects increases, the number of possible
arrangements also increases thereby increasing the
entropy of the system. The entropy increase, when n –
vacancies are formed, is given by
S = k log W …(3)
where W is the number of distinct ways in which ‘n’
indistinguishable vacancies can be produced (or
distributed) along N total number of sites per mole and is
given by  N! 
W  Cn  
N
 …(4)
 ( N  n)! n!
and the different ways in which n vacancy pairs(since the
numbers of cation and anion vacancies are equal) can be
produced, is simply obtained by squaring the above
expression, i.e.,  N! 
2

W  Cn  Cn  
N N
 …(5)
 ( N  n)! n!
using equations (2), (3) and (5) in equation (1) we have
2
 N! 
F  U  TS  n E  T  k log 
  …(6)
 ( N n )! n!

Using Stirling’s approximation for x >>1, we can write

log x!  x log x  x
2
 N!   N! 
log    2 log    2 log N! log ( N  n)!  log n!
 ( N  n)! n!  ( N  n)! n!

 2 N log N  N  ( N  n) log( N  n)  ( N  n)  n log n  n

 2 N log N  ( N  n) log( N  n)  n log n
Using this in equation (6), we have
F  nE  2 kT N log N  ( N  n) log( N  n)  n log n …(7)
Although internal energy increases, the effect of increase in
entropy finally results in the decrease of free energy(see
equation 1). When addition of more defects no longer
lowers the value of free energy F, the equilibrium state of
minimum(or constant) free energy has reached for a given
temperature. At this stage
 dF  …(8)
   0
 dn  T
d
nE  2 kT N log N  ( N  n) log( N  n)  n log n   0
dn
Solving the differentials :
d
nE   1  E  E …(i)
dn
d
N log N  ( N  n) log( N  n)  n log n
dn
  1   1   1 
 0  N  n   log N  n    1  n   log n  1
  N n   n 
   N  n  1  
    log N  n    1  1  log n
  N n  
 1  log N  n  1  log n

 log N  n  log n …(ii)

Therefore, using these differentials (i) and (ii), equation
(8) becomes
 F 
   E  2 k T log( N  n)  log n  0 …(9)
 n T
 N  n
 E  2 kT log 
 n 
 N  n E
 log  
 n  2kT

N n E
  e 2 kT
n
As the number of vacancies in a crystal is much smaller
than the number of ions, i.e., n N and (N – n)  N,
therefore, above equation becomes

N E
 e 2 kT
n
n 1 E
  E  e 2 kT
N e 2 kT
 n  Ne
E
2 kT …(10)

= concentration of Schottky defects

Therefore, we can conclude that the number of
Schottky defects in a crystal depends upon :

• the total number of ion – pairs(N),

• the average energy(E) required to produce a
Schottky defect and
• temperature(T).

It is also found that the fraction of Schottky

defects increases exponentially with increasing
temperature and a certain amount of defects
are always present at all temperatures above
the absolute zero.
Concentration of Frenkel Defects

For concentration of Frenkel defects, let us again consider a

pure crystal consisting of equal numbers of positively and
negatively charged ions. Consider that it contains a total of
N ions and n Frenkel defects, i.e., n – cation / anion
vacancies and n – interstitial ions in its interior.
Now Helmholtz free energy F is defined as
F = U – T.S …(1)
here U is the internal energy, S is the entropy and T is the
temperature.
If Ei as the energy required to create a vacancy, i.e., to
displace a cation from its regular position to an interstitial
occupation, then increase in internal energy for n Frenkel
defects is given by
U = nEi …(2)
 The entropy increase is given by
S = k log W …(3)
where W is the number of distinct ways in which ‘n’
Frenkel defects can be produced and is given by
N! Ni !
W  …(4)
( N  n)!n! ( N i  n)! n!
here Ni is the number of interstitial positions in the crystal.
 N! Ni ! 
 S  k log    …(5)
 ( N  n)! n! ( N i  n )! n!
Now Helmholtz free energy(equation 1) becomes

 N! Ni ! 
F  n Ei  k T log    …(6)
 ( N  n)! n! ( N i  n )! n!
Solving the factorial terms by using Strirling’s
approximation, for x1
log x!  x log x  x
we get
 N! Ni !   N log N  N i log N i  ( N  n) log( N  n)
log    
 ( N  n)! n! ( N i  n )! n!   ( N i  n) log( N i  n)  2n log n 
Putting this value in equation (6), we have
 N log N  N i log N i  ( N  n) log( N  n)
F  nEi  kT   …(7)
  ( N i  n) log( N i  n)  2n log n 
When equilibrium is reached, the free energy is minimum,
i.e., at equilibrium
 dF 
   0 …(8)
 dn  T
  N log N  N i log N i  ( N  n) log( N  n)

d
nEi   kT d  ( N  n) log( N  n)  2n log n  0
dn dn  i i 
solving the differentials, we will get
 dF   ( N  n)( N i  n)  …(9)
   Ei  k T log   0
 dn  T  
2
n
As N  n and Ni  n, therefore, the above equation becomes
Ei  N Ni 
 log  2   log( N N i )  2 log n
kT  n 
1 Ei
log n  log( N N i ) 
2 2kT
1  Ei
n  (N Ni ) 2
e 2 kT …(10)
= concentration of Frenkel defects
Therefore, in this case we find that the number of
Frenkel defects in a crystal depends upon :
(i) the product (NNi)1/2,
(ii) the average energy Ei and
(iii) temperature (T).

It can also be concluded that number of defects in a

crystal are found to increase exponentially as its
temperature rises. It is also found that certain amount
of defects are always present in a crystal at all
temperatures above absolute zero.