New insight in the uranium valence state determination in UyNd1−yO2±x

The charge compensation mechanisms in U y Nd 1− y O 2± x and its consequence on the overall O stoichiometry (or O/M ratio where M = Nd + U) have been studied through the uranium valence state mixture evolution as a function of Nd content up to y = 0.62 by means of high energy resolution fluorescence detection X-ray absorption spectroscopy (HERFD-XAS) at the U M 4 -edge. Our results clearly demonstrate the formation of U 5+ at low Nd content (y 0.15). Upon increasing the Nd content, oxygen vacancies and the formation of U 6+ appear as competing mechanisms for intermediate Nd concentrations, leading to the co-existence of U 4+ /U 5+ /U 6+ mixed valence and an overall hypostoichiometry (O/M 2.00). Finally, the formation of U 6+ associated with strongly distorted U local environment is observed for high Nd concentrations (y = 0.62), leading to an overall hyperstoichiometry (O/M 2.00).