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

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