Thermodynamics of CoAl2O4–CoGa2O4 solid solutions

CoAl2O4, CoGa2O4, and their solid solution Co(GazAl1−z)2O4 have been studied using high temperature oxide melt solution calorimetry in molten 2PbO·B2O3 at 973 K. There is an approximately linear correlation between lattice parameters, enthalpy of formation from oxides, and the Ga content. The experimental enthalpy of mixing is zero within experimental error. The cation distribution parameters are calculated using the O’Neill and Navrotsky thermodynamic model. The enthalpies of mixing calculated from these parameters are small and consistent with the calorimetric data. The entropies of mixing are calculated from site occupancies and compared to those for a random mixture of Ga and Al ions on octahedral site with all Co tetrahedral and for a completely random mixture of all cations on both sites. Despite a zero heat of mixing, the solid solution is not ideal in that activities do not obey Raoultʹs Law because of the more complex entropy of mixing.