Engineering Chemistry S Datta

SOLID STATE CHEMISTRY 491
equal number of cationic and anionic vacancies would exist in the crystal lattice as
shown in Fig. 22.15 below. This type of crystal defect is known as Schottky defect.
Such defect is found in crystals of NaCl.
(ii) Frenkel Defects. Frenkel defects arise with those crystals in which an ion leaves
its normal lattice site and occupies an interstitial site in the same crystal. For example,
in AgBr crystal some Ag + ions are missing from their regular positions and are
found to be squeezed between other ions.
+ – + – + –
– + – + – +
+ – + – + –
– + – + – +
+ – + – + –
– + – + – +
+ – + – + –
– – + – +
+ – + + –
– + – + – +
+ – + – + –
– + – + – +
+ – + – + –
+
– – + – +
+ – + – + –
– + – + – +
+ – + – + –
– + – + – +
Ideal Crystal Schottky defect Frenkel defect
Fig. 22.15 Defects in crystals.
Non-stoichiometric defects
Besides structural imperfections, crystal defects may also arise as a result of the presence
of small amount of impurities. These impurities in crystalline substance drastically change
their properties. For example, if a very small amount of CaCl 2
(less than 0.1%) is added to
NaCl, the conductivity of NaCl increases by 10000 times. In the mixed crystal Ca 2+ ions occupy
the positions of Na + in crystal lattice and the position of Cl – ion in the lattice remains unchanged.
The insertion of Ca 2+ ions for Na + ions leads to create lattice vacancies and such vacancies
permit the migration of ions as a result of which the conductivity of impure crystal increases.
One of the most common examples for impurity defect is semiconductors. Impurity defects are
introduced under controlled conditions in germanium and silicon in the production of
semiconductors.
In compounds having non-stoichiometric defects, the ratio of positive and negative ions
differs from that indicated in their respective chemical formula and the balance of the positive
and negative charges is maintained by extra electron or positive ions as per necessity. There is
an excess of either metal or non-metal atoms in non-stoichiometric compounds. Thus two cases
generate (i) Metal excess and (ii) Metal deficiency.
Metal excess may occur in the following ways:
(a) an anion may be missing from its lattice site and an electron be present there for
maintaining charge balance. For example: when NaCl is treated with sodium vapour,
such a yellow non-stoichiometric variety is obtained.
(b) an extra metal atom may be present in an interstitial lattice site and an electron
being present in some other interstitial position balances the charge. ZnO exhibits
this type of defect when its composition becomes Zn 1+x
O. The free electron present is
responsible for excitation to higher energy level by absorption of radiation of particular
wavelengths. This phenomenon occurs when ZnO is subjected to heating and its
colour changes from white to yellow.
Metal deficiency may also occur as follows:
A positive ion may be missing from its lattice position and a doubly charged cation
maintains the charge balance. FeO, FeS and NiO are the examples of such defect.