Enthalpies of formation of Ce-pyrochlore

High temperature oxide melt solution calorimetry was used to derive standard enthalpies of formation, ΔH_f⁰ (kJ/mol), for three pyrochlore phases: Ca_0.93Ce_1.00Ti_2.035O_7.00 (-3656.0 ± 5.6), Ca_1.46U_0.23⁴⁺U_0.46⁶⁺T _1.85O_7.00 (-3610.6 ± 4.1) and Gd₂Ti₂O₇ (-3822.5 ± 4.9). Enthalpy of drop solution data, ΔH_ds, were used to calculate enthalpies of formation with respect to an oxide phase assemblage, ΔH_f-ox⁰: CaO + MO₂ + 2TiO₂ = CaMTi₂O₇ or Gd₂O₃ + 2TiO₂ = Gd₂Ti₂O₇, and an oxide/perovskite phase assemblage, ΔH_f-pv+ox⁰: CaTiO₃ + MO₂ + TiO₂ = CaMTi₂O₇, where M = Ce or U. All three pyrochlore samples were stable in enthalpy relative to an oxide assemblage with ΔH_f-ox⁰ (kJ/mol) (Gd₂Ti₂O₇) = -113.4 ± 2.8; ΔH_f-ox⁰(Ca_1.46U_0.23⁴⁺ U_0.46⁶⁺Ti_1.85O_7.00 = -123.1 ± 3.4; ΔH_f-ox⁰(Ca_0.93Ce_1.00Ti_{2.0 35}O_7.00 = - 54.1 ± 5.2. U-pyrochlore was stable in enthalpy relative to an oxide/perovskite assemblage (ΔH_f-pv+ox⁰ = -5.1 ± 4.0 kJ/mol). Ce-pyrochlore was metastable in enthalpy relative to the oxide/perovskite phase assemblage (ΔH_f-pv+ox⁰ = +21.0 ± 5.5 kJ/mol). A significant metastability field was defined with respect to an oxide/perovskite phase assemblage. However, the proposed waste form baseline composition lies in the stable regions of the phase diagrams.