Co3O4-Co2ZnO4 spinels: The case for a solid solution

► Experiments confirm theory prediction of a solid solution in Co3O4-Co2ZnO4 spinels. ► Thermodynamics of mixing studied by calorimetric, electrical, structural methods. ► Cation distributions found to be mostly normal for various compositions. ► Mixing enthalpy is larger than prediction and inconsistent with strain effects. ► Changes in Co3+ spin state proposed as source of large mixing enthalpy/entropy.