Enthalpies of formation of U-, Th-, Ce-brannerite: implications for plutonium immobilization

Brannerite, ideally MTi2O6, (M = actinides, lanthanides and Ca) occurs in titanate-based ceramics proposed for the immobilization of plutonium. Standard enthalpies of formation, DeltaH(f)(0) at 298 K, for three brannerite compositions (kJ/ mol): CeTi2O6 (-2948.8 +/- 4.3), U0.97Ti2.03O6 (-2977.9 +/- 3.5) and ThTi2O6 (-3096.5 +/- 4.3) were determined by high temperature oxide melt drop solution calorimetry at 975 K using 3Na(2)O . 4MoO(3) solvent. The enthalpies of formation were also calculated from an oxide phase assemblage (DeltaH(f-ox)(0) at 298 K): MO2 + 2TiO(2) = MTi2O6, Only UTi2O6 is energetically stable with respect to an oxide assemblage: U0.97Ti2.03O6 (DeltaH(f-ox)(0) = -7.7 +/- 2.8 kJ/mol). Both CeTi2O6 and ThTi2O6 are higher in enthalpy with respect to their oxide assemblages with (DeltaH(f-ox)(0) = +29.4 +/- 3.6 kJ/mol) and (DeltaH(f-ox)(0) = +19.4 +/- 1.6 kJ/mol) respectively. Thus, Ce- and Th-brannerite are entropy stabilized and are thermodynamically stable only at high temperature. (C) 2003 Elsevier B.V. All rights reserved.