Thermodynamics of formation of chromite spinels

The enthalpies of formation from the oxides of the chromite spinels formed by Mg, Ni, Cu, Zn, Cd have been determined by oxide melt solution calorimetry in 2PbO·B 2 O 3 melts at 900°C. The values are somewhat more exothermic but otherwise largely parallel those of the corresponding aluminates, ferrites, manganites, and gallates. For some of the chromites reliable Gibbs (free) energies of formation have been reported in the literature. A comparison with the new enthalpy data yields corresponding entropy information. For NiCr 2 O 4 and CuCr 2 O 4 the entropies of formation are large and positive, consistent with a high degree of cation disorder.