The compositional dependence of the concentrations of the majority point defects (cation vacancies and cation interstitials) was studied in spinels at 1200C, under a constant O partial pressure. It was found that the energies of point defect formation in the oxide spinel solid solutions depended upon the temperature and chemical composition, and could be related to so-called defect constants. The values of such constants were deduced by fitting experimental non-stoichiometry data to equations which were derived from point defect thermodynamics. The applicability of 2 different approaches to the modelling of the compositional dependence of the defect constants was investigated. The first approach was a model which assumed that the equilibrium constants of defect formation reactions in Fe3O4 were the same in Fe-based spinel solid solutions. This model worked quite well in the case of Co-Fe oxide spinels; especially at low Co concentrations. It did not work well for Fe-Mn spinel solid solutions. The other approach related the defect constants to the standard free energies for the oxidation, to spinels, of oxide phases with the rock-salt structure. This model worked for all of the spinel compositions which were investigated.

F.H.Lu, R.Dieckmann: Journal of the Physics and Chemistry of Solids, 1995, 56[5], 725-33