Enthalpies of formation of Ce-pyrochlore, Ca0.93Ce1.00Ti2.035O7.00, U-pyrochlore, Ca1.46U4+0.23U6+0.46Ti1.85O7.00 and Gd-pyrochlore, Gd2Ti2O7: three materials relevant to the proposed waste form for excess weapons plutonium

Abstract

High temperature oxide melt solution calorimetry was used to derive standard enthalpies of formation, ΔH⁰_f (kJ/mol), for three pyrochlore phases: Ca_0.93Ce_1.00Ti_2.035O_7.00 (−3656.0±5.6), Ca_1.46U⁴⁺_0.23U⁶⁺_0.46Ti_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, ΔH⁰_f−ox: CaO+MO₂+2TiO₂=CaMTi₂O₇ or Gd₂O₃+2TiO₂=Gd₂Ti₂O₇, and an oxide/perovskite phase assemblage, ΔH⁰_f−pv+ox: CaTiO₃+MO₂+TiO₂=CaMTi₂O₇, where M=Ce or U. All three pyrochlore samples were stable in enthalpy relative to an oxide assemblage with ΔH⁰_f−ox (kJ/mol) (Gd₂Ti₂O₇)=−113.4±2.8; ΔH⁰_f−ox(Ca_1.46U⁴⁺_0.23U⁶⁺_0.46Ti_1.85O_7.00)=−123.1±3.4; ΔH⁰_f−ox(Ca_0.93Ce_1.00Ti_2.035O_7.00)=−54.1±5.2. U-pyrochlore was stable in enthalpy relative to an oxide/perovskite assemblage (ΔH⁰_f−pv+ox=−5.1±4.0 kJ/mol). Ce-pyrochlore was metastable in enthalpy relative to the oxide/perovskite phase assemblage (ΔH⁰_f−pv+ox=+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.