Ion-exchange equilibria between (Mn, Co)O solid solution and (Mn, Co) Cr₂O₄ and (Mn, Co) Al₂O₄ spinel solid solutions at 1100°C

Abstract

The compositions of the (Mn,Co)O solid solution with rock salt structure in equilibrium with (Mn,Co)Cr₂O₄ and (Mn,Co)Al₂O4 spinel solid solutions have been determined by X-ray diffraction measurements at 1100° C and an oxygen partial pressure of 10^-10 atm. The ion exchange equilibria are quantitatively analysed, using values for activities in the (Mn,Co)O solid solution available in the literature, in order to obtain activities in the spinel solid solutions. The MnAl₂O₄-CoAl₂O₄ solid solution exhibits negative deviations from Raoult's law, consistent with the estimated cation disorder in the solid solution, while the MnCr₂O₄-CoCr₂O₄ solid solution shows slightly positive deviations. The difference in the Gibbs free energy of formation of the two pure chromites and aluminates derived from the results of this study are in good agreement with recent results obtained from solid oxide galvanic cells and gas-equilibrium techniques.