Impurities and their Effects on Defect Equilibria

Impurities introduced into the crystal lattice can strongly affect the defect equilibria [9, 15-19]. This influence can be illustrated by the example of MO oxide matrix doped with Me2O3, assuming that predominant defects in pure MO are vacancies in the metal and oxygen sublattices (Vm, V ) in accordance with Equation (3.30). The dissolution of Me2O3 is also assumed to occur via a substitution mechanism, that is, Me atoms incorporate into the M-sublattice  

Since the oxidation state of Me + is higher than that of Me +, the former acquires an effective positive charge. According to the structure conservation condition, an appearance of three new sites of oxygen must be accompanied by the formation of an equal number of the metal sites, with two sites occupied by Me + ions and one site vacant. The overall electroneutrahty condition in this case should be written as 

The so-called intrinsic region, where the concentration of impurities is relatively small and the content of thermally induced vacancies is, in contrast, relatively high. In this case, the electroneutrahty condition can be approximated by 

The so-called impurity-control region, where the opposite situation takes place and the electroneutrahty condition can be simplified to [Me jj] = 2[Vm]. 

---