Activity–Composition Relations in MnCr ₂ O ₄ –CoCr ₂ O ₄ Solid Solutions and Stabilities of MnCr ₂ O ₄ and CoCr ₂ O ₄ at 1300°C

Phase equilibria in the system MnO–CoO–Cr ₂ O ₃ were investigated at 1300°C under controlled oxygen partial pressures by using the gas equilibration technique. The CoO activities in various phase assemblages of the system were measured by determining the partial pressures of oxygen in the gas phase for coexistence with metallic cobalt. The activity data revealed that at 1300°C, MnO–CoO and MnCr ₂ O ₄ –CoCr ₂ O ₄ solid solutions exhibit mild positive departures from ideal behavior. The activities in the stoichiometric spinel solutions were found to be in good agreement with those predicted from a model based on cation distribution equilibria. The standard free energy of formation of the compound CoCr ₂ O ₄ from its oxide components at 1300°C was determined as −37 636 J/mol, while that for MnCr ₂ O ₄ was found as −44 316 J/mol.