Evolution of the uranium chemical state in mixed-valence oxides

The chemical states of uranium in the binary oxides UO2, U4O9, U3O7, U3O8, and UO3 are a fundamental yet hitherto unresolved question, which we solve here by using High Energy Resolution Fluorescence Detection X-ray Absorption Near Edge Spectroscopy (HERFD-XANES) at the uranium M4 edge. In the mixed-valence oxides U4O9, U3O7, and U3O8 we were able to resolve the distribution of formal oxidation states. Moreover, we clearly identify a pivot from U(IV)−U(V) to U(V)−U(VI) charge compensation, corresponding to the transition from a fluorite-type structure (U3O7) to a layered structure (U3O8).[1] 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.