Thermodynamics of A₃O₄-B₃O₄ spinel solid solutions

The free energies of formation of solid solutions in the systems Fe₃O₄-Co₃O₄, Mn₃O₄-Co₃O₄, and Fe₃O₄-Mn₃O₄ have been described by a rather simple approach to the oxidation-reduction and cation distribution equilibria involved. Oxidation-reduction enthalpies for ions on octahedral sites in spinels were calculated by considering the thermodynamic relations between rocksalt and spinel binary oxides. These calculated enthalpies result in fairly good agreement with the experimental thermodynamic data and also agree with values of oxidation-reduction enthalpies estimated from defect equilibria in the spinels, i.e., from diffusion and electrical measurements. This suggests that such enthalpies may indeed remain relatively constant despite some variation of the environment in which the octahedrally coordinated cations are found.