Co3O4–Co2ZnO4 spinels: The case for a solid solution

In prior first-principles theoretical work we predicted a complete solid solution in the Co3O4–Co2ZnO4 system, with a negligibly small mixing enthalpy. In this work we tested this prediction on bulk, large-grained specimens across the Co3O4–Co2ZnO4 join, combining oxide melt solution calorimetry, differential scanning calorimetry, precise lattice parameter measurements, anomalous X-ray and neutron diffraction, and in situ electrical measurements. The calorimetric results confirm the presence of a solid solution at high temperatures, but with a large enthalpy of mixing that exceeds the predicted value. Because Co3O4 and Co2ZnO4 have essentially identical lattice parameters, this energetic destabilization must arise from factors other than the strain energy resulting from size mismatch. Changes in Co3+ spin states vs. temperature and zinc content are proposed to account for the positive excess enthalpy, and may also provide additional entropy to stabilize the solid solution at high temperature.