Point defects of compounds

From consideration of the point defects of elements mentioned in Section 1.3.1, the nature of the point defects of compounds MX, where M is metal and X is O, S, N, H (volatile elements) etc., can be easily understood. Possible point defects of the compounds MX are as follows  

M(X) or X(M) metal on anion site or anion on metal site (substitution) 

These symbols, which will be used hereafter in this book, are after Kroger and Vink (see Fig. 1.9). By combinations of these defects, a variety of defect structure types is produced. If both the combination and the concentration of point defects are adequate, the composition of a compound is stoichiometric, but this is a special case, and compounds with point defects are usually non-stoichiometric compounds. Typical examples of defect structure types are  

Frenkel type in which metal atoms on regular sites move to interstitial sites, leaving metal vacancies, i.e. (M = MJ (Fig. 1.9(b)). Anti-Frenkel type defects, in which anion atoms on regular sites move to interstitial sites, are also possible, but are rarely observed because the ionic radii of anions are usually larger than those of the metals under consideration. Frenkel type is stoichiometric. 

Schottky type in which equal numbers of M, and X are created (Fig. 1.9(c)), in this case, the composition is also stoichiometric, and anti-Schottky type, in which equal numbers of metal atoms and anions on respective regular sites move to respective interstitial lattice points leaving vacancies and X . 

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Fig. 1.9 Point defects of compounds (a) Various point defects, (b) Frenkel type defects, (c) Schottky type defects.

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