Defect chemistry of uranium oxides

Abstract

Uranium oxides are known as nonstoichiometric compounds whose composition changes according to external conditions such as temperature and oxygen partial pressure. The change of composition caused by the formation of defect structure results in a change of their properties. In this paper, the compositional changes of UO2 and doped UO2 [(U, M)O2; M=La, Ti, Pu, Th, Nb, Cr, etc.] and also those of other uranium oxides (U4O9, U3O8) are shown against oxygen partial pressure. From the results of doped UO2, it is concluded that the valence control rule holds to a first approximation. The defect structures are estimated both from log x vs. log Po2 (x: deviation from the stoichiometric composition and Po2: oxygen partial pressure) and log σ vs. log Po2 (σ: electrical conductivity) relations. The defect structures of UO2 and doped UO2 are derived based on the Willis model for UO2+x. The detect structure of U4O9 phase is similar to that of UO2+x, but the defect structures of U3O8 phase are complicated due to the existence of many higher-order phase transitions. The thermodynamic data such as the partial molar enthalpy and entropy and the heat capacity are important to characterize the defect structure. The high temperature heat capacities of UO2 doped with Gd show pronounced increases at high temperatures the onset temperature decreases as the dopant content increases. The increase of heat capacity is interpreted to be due to the formation of lattice defects. The heat capacity measurements on U4O9 and U3O8 clucidate the presence of the phase transition. The mechanisms of these phase transitions are discussed.