Crystal Defects

Crystals are rarely found to be perfect Atoms vibrate due to thermal effect and electrons change their position Type of defects
1. 2. 3. 4. Point defects Line defects Surface defects Volume defects

Point defects
The defects that take place due to imperfect packing of atoms during crystallisation, are known as point defects Point defects also take place due to vibration of atoms at high temperatures. Types of point defects are 1.Vacancies 2.Interstitial defects 3.Frenkel defect 4.Substitutional defect 5.Schottky defect 6.Phonon

Vacancy Defect
Whenever one or more atoms are missing from a normally occupied position, as in fig below defects caused is known as vacancy.

Vacancy Defect
Vacancy defects can be a result of imperfect packing during the formation of crystals. They also arise from thermal vibration of the atoms at high temperatures. It may be noted that there may be a single vacancy, di vacancy, tri vacancy and so on

Interstitial Defects
Whenever extra atom occupies interstitial position in the crystal system, without dislodging the parent atom the defect caused is called as interstitial defect.

Interstitial defects
1. The atom that occupies the interstitial position is generally smaller than the parent atom. 2. In close packed structure the largest size of atom that can fit in the voids have radius 22.5% of radii of parent atom. 3. If atom of larger size fits into the voids they produce distortion in atoms.

Interstitial Defects

Point defects

Frenkel defect
When a missing atom (responsible for vacancy) occupies interstitial position (responsible for interstitial defect) the defect caused is frenkel defect

Substitutional defect
Foreign atom (i.e. other than the parent atom) occupies a position, which was initially meant for parent atom. The size of replaced atom may not be same as parent atom

Schottky defect
If a pair of +ve and ve ions is missing from a crystal, it is schottky defect. The crystal is electrically neutral.

Point defects
(1) vacancy (2) self-interstitial (3) interstitial impurity (4) Small Substitutional impurity (5) Large substitutional impurity.

Phonon
Whenever a group of atoms is displaced from its ideal location, the defect is known as phonon Such defect is caused by thermal vibrations. When atoms interact with one another they vibrate in synchronism like the waves on ocean. This defect affects the electrical and magnetic properties

Line defects
The defects that take place due to dislocation or distortion of atoms along a line, in some direction, are called as line defects. Line defects also take place when a central portion of a crystal lattice slips without effecting the outer portion. Types of Line defects:1.Edge dislocation 2.Screw dislocation 3.Mixed dislocation

Edge dislocation
When half plane of atom is inserted between the planes of atoms in a perfect crystal, the defect is edge dislocation

Edge dislocation

Dislocation line

+ve edge dislocation

-ve edge dislocation

Symbolically +ve edge dislocation is represented as and the - ve edge dislocation is represented as

Edge Dislocation
If extra half plane in the upper plant of the crystal, atoms in the upper half experience compressive stress & atoms in the bottom half experience tensile stress. The dislocation line is simply the edge of extra half plane in the crystal and hence the name of edge dislocation

Edge Dislocation Glide

Edge Climb

Positive climb Removal of a row of atoms

Negative climb Addition of a row of atoms

Edge Dislocation

Perfect crystal

Edge Dislocation
Extra Half plane of atoms Burgers or Slip vector

Edge Dislocation
Crystal with edge dislocation
Burgers Vector

Burgers circuit
A dislocation in a crystal can be identified by considering an atom-to-atom path known as Burgers circuit around the dislocation line A burger circuit always closes in a perfect crystal & in an imperfect crystal containing a dislocation, the circuit fails to close . The vector that is required to close the circuit around a dislocation line is known as Slip vector or Burgers vector (closure-failure distance) & is denoted by The burgers vector , describes both the magnitude & direction of dislocation

 If an imaginary circuit is drawn, atom to atom around a dislocation line, an extra step is needed to complete the circuit, compared to a similar circuit in a perfect crystal. This extra step is called the Burgers vector or slip vector. An edge dislocation is one that has its Burgers vector at right angles to the dislocation line

In a screw dislocation, the Burgers vector is parallel to the dislocation line, and the arrangement of atoms around the dislocation is like a spiral staircase

Screw dislocation
When 2 separate planes are shifted due to application of shear force we get screw dislocation The arrangement of atoms appear like a that of a screw or helical surface.

Screw Dislocation

Dislocation Line

Burgers vector b

Screw dislocation from above

Mixed dislocations

Pure screw

Pure Edge

Edge dislocation

Screw Dislocation

Burger vector b is perpendicular to Burger vector b is parallel to dislocation line dislocation line
Number of atoms involved in edge dislocation are less Atoms involved in formation of screw dislocation are more

Force required for formation & movement of edge dislocation is less relatively

Relatively more force is required in formation & movement of screw by dislocations

Edge dislocation may be +ve or - Screw dislocation may be right ve handed or left handed High speed of movement of edge dislocation An edge dislocation moves in the direction of Burgers vector (slip direction) in the slip plane. Speed of movement is less A screw dislocation moves in a direction to Burgers vector (slip direction) in the slip plane

Surface defects
Defects that take place on the surface of a material are known as surface defects or plane defects. Surface defects take place either due to imperfect packing of atoms during crystallisation or defective orientation of the surface. Types of surface defects
1. 2. 3. Grain boundary Twin boundary Stacking fault

Grain Boundary
Whenever grains of different orientation separate the general pattern of atoms and exhibits a boundary the type of defect is grain boundary. It usually takes place during solidification of liquid metal

Grain Boundary

Twin boundary
When the boundaries in which the atomic arrangement on one side of the boundary is mirror image of the arrangement of atoms of the other side defect is called as twin boundary The region in which a twin boundary defect occurs is between the twinning planes.

Stacking fault
When the stacking of atoms is not in proper sequence throughout the crystal the fault is called as stacking fault. The region where stacking fault occurs forms a thin region of hexagonal close packing in a FCC crystal.

Volume defects
1. Pores - can greatly affect optical, thermal, mechanical properties 2. Cracks - can greatly affect mechanical properties 3. Foreign inclusions - can greatly affect electrical, mechanical, optical properties

Why study crystal defects.?

Effects of defects on properties of materials

Line defects or Dislocations influence the mechanical properties of crystalline materials. The modification of mechanical properties (such as strength level, ductility, toughness) is closely related to dislocation theory.

Effects of defects on properties of materials