THERMODYNAMICS OF CoAl2O4-CoGa2O4 SOLID SOLUTIONS

CoAl₂O₄, CoGa₂O₄ and their solid solution CoGa₂-_2NAl_2NO₄ have been studied, using high temperature oxide melt calorimetry in molten 2PbO.B₂O₃ solvent at 973 K. There is an approximately linear correlation between lattice parameters, enthalpy of formation from oxides and the Ga content in the samples. The solid solution CoGa₂-_2NAl_2NO₄ shows an enthalpy of mixing which is zero within the experimental errors. The cation distribution parameters are calculated at 673, 773 and 873 K, using the O'Neill and Navrotsky thermodynamic model. The entropies of mixing of solid solution are calculated from site occupancies and for a random mixture of Ga and Al ions on octahedral site and compared. Despite a zero heat of mixing, the solid solution is not ideal in that activities do not obey Raoult’s Law.