Enthalpy of formation of the cubic fluorite phase in the ceria–zirconia system

The enthalpy of formation of cubic ceria–zirconia solid solutions (c-Ce_(1−x)Zr_xO₂, 0.05 ⩽ x ⩽ 0.75) at 25 °C with respect to monoclinic zirconia (m-ZrO₂) and cubic ceria (c-CeO₂) has been measured by high-temperature oxide melt solution calorimetry. In contrast to fluorite solid solutions containing trivalent oxides (e.g., yttria–zirconia), mixing in c-Ce_1−xZr_xO₂ shows moderate positive deviation from ideality. Evaluating the data within the framework of a regular solution model, the interaction parameter, Ω, is +51.0 ± 8.0 kJ/mol. The introduction of undersized Zr into CeO₂ severely distorts and destabilizes the oxygen sublattice. Destabilization of c-Ce_1−xZr_xO₂ may be relieved by reduction or clustering. A stable ordered compound in the CeO₂–ZrO₂ system is thermodynamically unlikely.