The enthalpy of formation of cubic yttria-stabilized hafnia from monoclinic hafnia and C-type yttria was measured by oxide melt solution calorimetry. The enthalpies of formation fit a function independent of temperature and quadratic in composition. The enthalpies of transition from m-HfO 2 and C-type YO 1.5 , to the cubic fluorite phase are 32.5 ± 1.7 kJ/mol and 38.0 ± 13.4 kJ/mol, respectively. The interaction parameter in the fluorite phase is strongly negative, −155.2 ± 10.2 kJ/mol, suggesting even stronger short range order than in ZrO 2 -YO 1.5 . Regular solution theory or any other model assuming random mixing on the cation and /or anion sublattice is not physically reasonable. A more complex solution model should be developed to be consistent with the new calorimetric data and observed phase relations.

Solid solutions (1 − x)CeO2 − xYO1.5 (0 ≤ x ≤ 0.36) were prepared by coprecipitation and sol-gel methods. Their enthalpy of formation relative to the end-members, fluorite-type cubic CeO2 and C-type YO1.5 was determined by oxide melt solution calorimetry. The enthalpy of drop solution shows a roughly linear trend with composition. Extrapolation to x = 1 gives the transition enthalpy of C-type to cubic fluorite YO1.5 as 22.2 ± 6.7 kJ/mol. This linear behavior is in contrast to the strong curvature seen in the ZrO2 − YO1.5 and HfO2 − YO1.5 systems. The slightly positive enthalpy of formation of CeO2 − YO1.5 is strikingly different from the strongly negative enthalpies of formation of ZrO2 − YO1.5 and HfO2 − YO1.5. The thermodynamics of CeO2 − YO1.5 is analyzed in terms of defect association and oxygen vacancy distribution. Specifically, the association of oxygen vacancies with the tetravalent cations in the zirconia and hafnia systems, in contrast to the preference of vacancies for nearest neighbor yttrium sites in the ceria systems, may explain the different energetics.

The enthalpy of formation of cubic ceria–zirconia solid solutions (c-Ce(1−x)ZrxO2, 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-Ce1−xZrxO2 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-Ce1−xZrxO2 may be relieved by reduction or clustering. A stable ordered compound in the CeO2–ZrO2 system is thermodynamically unlikely.

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