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
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.
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.