Evolution of the Uranium Chemical State in Mixed-Valence Oxides

A fundamental question concerning the chemical state of uranium in the binary oxides UO₂, U₄O₉, U₃O₇, U₃O₈, and UO₃ is addressed. By utilizing high energy resolution fluorescence detection X-ray absorption near edge spectroscopy (HERFD-XANES) at the uranium M₄ edge, a novel technique in the tender X-ray region, we obtain the distribution of formal oxidation states in the mixed-valence oxides U₄O₉, U₃O₇, and U₃O₈. 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 (U₃O₇) to a layered structure (U₃O₈). 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.