Thermochemistry of rare earth doped uranium oxides Ln_xU_1-xO_2-0.5x+y (Ln = La, Y, Nd)

Abstract Lanthanum, yttrium, and neodymium doped uranium dioxide samples in the fluorite structure have been synthesized, characterized in terms of metal ratio and oxygen content, and their enthalpies of formation measured by high temperature oxide melt solution calorimetry. For oxides doped with 10-50 mol % rare earth (Ln) cations, the formation enthalpies from constituent oxides (LnO_1.5, UO₂ and UO₃ in a reaction not involving oxidation or reduction) become increasingly exothermic with increasing rare earth content, while showing no significant dependence on the varying uranium oxidation state. The oxidation enthalpy of Ln_xU_1-xO_2-0.5x+y is similar to that of UO₂ to UO₃ for all three rare earth doped systems. Though this may suggest that the oxidized uranium in these systems is energetically similar to that in the hexavalent state, thermochemical data alone can not constrain whether the uranium is present as U⁵⁺, U⁶⁺, or a mixture of oxidation states. The formation enthalpies from elements calculated from the calorimetric data are generally consistent with those from free energy measurements.