iii NonStoichiometric Defects The defects discussed so far

(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. Na. Cl, Li. Cl, 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. Zn. O.

F-center (e- trapped in anionic vacancy)

§ Zn. O heated in Zn vapour → Zny. O (y >1) § The excess cations occupy interstitial voids § The electrons (2 e ) 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.

§ Fe. O heated in oxygen atmosphere → Fex. O (x <1) § Vacant cation sites are present § Charge is compensated by conversion of ferrous to ferric ion: Fe 2+ → Fe 3+ + 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 (Fe 2+) vacancies