Point Defects and Departures from Stoichiometry

This last equation is equivalent to the reaction of La2Ti3C 9 (i.e., La2/3Ti03) with SrTi03. 

At low concentrations of the lanthanum dopant, the vacancies appear to be distributed at random over the available anion sites. However, as the concentration of La3+ and hence of vacancies increases, both tend to order in the crystal. 

Many binary inorganic phases with a significant composition range can be listed1 (Table 4.3). Apart from binary compounds, ternary and other more complex materials may show nonstoichiometry in one or all atom components. 

The simplest way to account for composition variation is to include point defect populations into the crystal. This can involve substitution, the incorporation of unbalanced populations of vacancies or by the addition of extra interstitial atoms. This approach has a great advantage in that it allows a crystallographic model to be easily constructed and the formalism of defect reaction equations employed to analyze the situation (Section 1.11). The following sections give examples of this behavior. 

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