Equilibria in Binary Compounds

In binary compounds the sublattice of both components may contain the point defects discussed in Section 1.2. In addition, interstitials of either component may be present. Any one of these point defects will give rise to an excess or deficiency of one of the components. Therefore, in order to maintain the stoichiometric composition, there must be at least two types of defects, having opposite effects on the stoichiometry, present in equivalent concentrations. 

Because of differences in defect formation energies, the concentrations of all other defects usually are negligible compared to the two predominant types. 

Let us consider a generalized binary compound MX, where M represents the more electropositive component, X the more electronegative component, and s the stoichiometric ratio of X to M. The nine possible ways that two types of defects may exist in the MX lattice are listed in Table 1. These conjugate pairs of defects are called 

Equations for the formation of each of these intrinsic defects may be formulated. For Schottky defects. 

The a interstices (in this case a is the number of interstices per M site) are generated because the total number of sites is increased on formation of Schottky defects. However, if no interstitial defects are present, the concentration of Vj with respect to metal atom sites is merely a. 

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