Defects in solids

Defects in solids

Imperfection in solids: defects in solids

Any departure from the perfectly ordered arrangement of atoms in crystal is called imperfections or defects.
An ideally perfect crystal is one which has the same unit cell and contains the same lattice points throughout the crystal. Imperfections not only modify the properties but also sometimes impart new properties to the solids.

Atomic imperfections / point defects:

When deviation occurs because of missing atoms, displaced atoms or extra atoms, the imperfection is celled a point defect. 
Type of point defects – point defects in a crystal may be classified into three types
·         Stoichiometric defects
·         Non – stoichiometry defects
·         Impurity defects

Stoichiometry defects

The defects that do not disturb the ratio of cations and anions are called Stoichiometric defect.
These are further classified into:
(a) Schottky defect
(b) Frankel defect

(a) Schottky defect: - It is a vacancy defect in ionic solids. No. of missing cations and anions are equal, so the electrical neutrality is maintained. This defect decreases the density of the substance.  This defect is shown by ionic substances in which cation & anion are of almost similar sizes. e.g. NaCl, KCl, AgBr. Etc.

(b) Frankel defect: In ionic solids the smaller ion is dislocated from its normal site to an interstitial site. It creates a vacancy defect at its original site and an interstitial defect at its new location. It does not change the density of the solid. This type of defect is shown by ion substances in which there is a large difference in the size of ions. eg. ZnS, AgCl, AgBr etc.

 Non – Stoichiometric defects 

If as a result of imperfection, the ratio of number of cation to anion becomes different from that indicated by the ideal chemical formula; the defects are called non – Stoichiometric defects.
These defects arise either due to excess of metal atoms or excess of non-metal atoms.
These are of 2 types:-
(a) Metal excess defect
(b) Metal deficiency defect

(a) Metal Excess Defects. The Colour Centres.

The anion may be missing from its lattice site leaving an electron behind so that the charge remains balanced. The sites containing the electrons are called as F-centres because they are responsible for imparting colour to the crystals; F stands for Farbenzenter meaning colour.

 It has been observed that if a crystal of NaCl is heated in sodium vapour, it acquires a yellow colour. This yellow colour is due to the formation of a non-Stoichiometric compound of sodium chloride in which there is a slight excess of sodium ions.
What happens in this case is that some sodium metal gets doped into sodium chloride crystal which, due to the crystal energy, gets ionized into Na+ and e.

(b) Metal Deficiency Defects

This type of defect is generally found amongst the compounds of transition metals which can exhibit variable oxidation state. In this defect cases, one of the positive ions is missing from its lattice site and the extra negative charge is balanced by some nearby metal ion acquiring two charges instead of one. There is evidently, a deficiency of the metal ions although the crystal as a whole is neutral. Crystals of FeO, FeS and NiO show this type of defects.
 It is evident from the above discussion that all types of point defects result in the creation of vacancies or ‘holes’ in the lattice of the crystals. The presence of holes lowers the density as well as the lattice energy or the stability of the crystals. The presence of too many holes may cause a partial collapse of the lattice.