Evolution of the Uranium Chemical State in Mixed-Valence Oxides.

A fundamental question concerning the chemical state of uranium in the binary oxides UO ₂ , U ₄ O ₉ , U ₃ O ₇ , U ₃ O ₈ , and UO ₃ is addressed. By utilizing high energy resolution fluorescence detection X-ray absorption near edge spectroscopy (HERFD-XANES) at the uranium M ₄ edge, a novel technique in the tender X-ray region, we obtain the distribution of formal oxidation states in the mixed-valence oxides U ₄ O ₉ , U ₃ O ₇ , and U ₃ O ₈ . Moreover, we clearly identify a pivot from U(IV)-U(V) to U(V)-U(VI) charge compensation, corresponding with transition from a fluorite-type structure (U ₃ O ₇ ) to a layered structure (U ₃ O ₈ ). Such physicochemical properties are of interest to a broad audience of researchers and engineers active in domains ranging from fundamental physics to nuclear industry and environmental science.