(iii) Non-Stoichiometric Defects
The defects discussed so far do not disturb the stoichiometry of the
crystalline substance. However, a large number of non-stoichiometric
inorganic solids are known which contain the constituent elements in nonstoichiometric ratio due to defects in their crystal structures.
These defects are of two types:
(a) Metal Excess Defect and (b) Metal Deficiency Defect
(a) Metal Excess Defect
Metal excess defect due to anionic vacancies: In this case, negative
ions may be missing from their lattice sites leaving holes in which the
electrons remain entrapped to maintain the electrical neutrality. E.g.
NaCl, LiCl, KCl,…
*The anionic sites occupied by unpaired electrons are called Fcentres (from the German word Farbenzenter for colour centre).
Metal excess defect due to the presence of extra cations at interstitial
sites: In this case, there are extra positive ions occupying interstitial
sites and the electrons in another interstitial sites to maintain electrical
neutrality. The defect may be visualised as the loss of non-metal atoms
which leave their electrons behind. The excess metal ions occupy
interstitial positions. E.g. ZnO.
F-center
(e- trapped
in anionic
vacancy)
 ZnO heated in Zn vapour → ZnyO (y >1)
 The excess cations occupy interstitial voids
 The electrons (2e) released stay associated to the interstitial cation
(b) Metal Deficiency Defect
(i) Cation Vacancies: In some cases, the positive ions may be
missing from their lattice sites.
The extra negative charge may
be balanced by some nearby metal ion acquiring two positive
charges instead of one. This type of defect is possible in metals
which show variable oxidation states.
The common examples of compounds having this defect are
ferrous oxide, ferrous sulphide, nickel oxide etc.
(ii) Extra anions occupying interstitial sites: In the case, the extra
anions may be occupying interstitial positions. The extra negative
charge is balanced by the extra charges on the adjacent metal ions.
Such type of defect is not common because the negative ions
usually very large and they cannot easily fit into the interstitial sites.
 FeO heated in oxygen atmosphere → FexO (x <1)
 Vacant cation sites are present
 Charge is compensated by conversion of ferrous to ferric ion:
Fe2+ → Fe3+ + e
 For every vacancy (of Fe cation) two ferrous ions are converted to
ferric ions → provides the 2 electrons required by excess oxygen
Metal Deficiency Defect due to Cationic (Fe2+) vacancies