Thermodynamic stability of the fluorite phase in the CeO ₂ − CaO − ZrO ₂ system

The fluorite phase in CeO ₂ − CaO − ZrO ₂ is important in nuclear energy and fuel cell applications yet one lacks quantitative evaluation of its composition range and thermodynamic properties. Using experimental thermodynamic data from binary systems ZrO ₂ − CeO ₂ CaO − CeO ₂ and CaO − ZrO ₂ , the present work fitted the enthalpies of mixing to regular solid solution models. The interaction parameters for binary systems are Ω _CeO2-ZrO2 = 49.9 ± 2.4 kJ mol ⁻¹ , Ω _CaO-ZrO2 = −78.0 ± 16.7 kJ mol ⁻¹ and Ω _CaO-CeO2 = 53.1 ± 21.5 kJ mol ⁻¹ . Enthalpies of transformation of monoclinic zirconia to the cubic fluorite phase (stable at high temperature) and of rocksalt calcia to the hypothetical fluorite phase were constrained by experimental data. Enthalpies of mixing in the ternary CeO ₂ − CaO − ZrO ₂ system were calculated from enthalpies of drop solution measured by high temperature oxide melt solution calorimetry for a set of newly synthesized ternary fluorite samples. Comparison of measured values with those predicted from the binary systems using the Kohler equation suggests that the ternary excess term is zero within experimental error. Thus simple equations could be derived for heats of formation, mixing and phase transition in this system which can be used in the future in CALPHAD type calculations of phase equilibria that obtain the extent of fluorite solid solution at different temperatures.