Enthalpies of formation of Ce-pyrochlore, Ca0.93Ce1.00Ti2.035O7.00, U-pyrochlore, Ca1.46U0.234+U0.466+Ti1.85O7.00 and Gd-pyrochlore, Gd2Ti2O7:: three materials relevant to the proposed waste form for excess weapons plutonium

High temperature oxide melt solution calorimetry was used to derive standard enthalpies of formation, AHf (kJ/mol), for three pyrochlore phases: Ca0.93Ce1.00Ti2.035O7.00 (-3656.0 +/- 5.6), Ca1.46U0.234+U0.466+Ti1.85O7.00 (-3610 +/- 4.1) and Gd2Ti2O7 (-3822.5 +/- 4.9). Enthalpy of drop solution data, DeltaH(ds), were used to calculate enthalpies of formation with respect to an oxide/perovskite phase assemblage AH(f-ox)(0): CaO + MO2 + 2TiO(2) = CaMTi2O7 or Gd2O3 + 2TiO(2) = Gd2Ti2O7, and an oxide/perovkite phas assemblage AH(f-pv+ox)(0): CaTiO3 + MO2 + TiO2 = CaMTi2O7, where M = Ce or U. All three pyrochlore samples were stable in enthalpy relative to an oxide assemblage with DeltaH(f-ox)(0) (kJ/mol) (Gd2Ti2O7) = -113.4 +/- 2.8; DeltaH(f-ox)(0)(Ca1.46U0.234+U0.466+Ti0.85O7.00) = -123.1 +/- 3.4; DeltaH(f-ox)(0)(Ca0.93Ce1.00Ti2.035O7.00) = -54.1 +/-5.2. U pyrochlore was stable in enthalpy relative to an oxide/perovskite assemblage (DeltaH(f-pv-ox)(0) = -5.1 +/- 4.0 kJ/mol). Ce-pyrochlore was metastable in enthalpy relative to the oxide/perovskite phase assemblage (DeltaH(f-pv-ox)(0) = +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. (C) 2002 Elsevier Science B.V. All rights reserved.