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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The MARS beamline at the SOLEIL synchrotron is dedicated to the characterization of radioactive material samples. One great advantage of the beamline is the possibility to characterize about 380 radionuclides by different X-ray techniques in the same place. This facility is unique in Europe. A wide energy range from around 3.5 keV to 36 keV K-edges from K to Cs, and L3 edges from Cd to Am and beyond can be used. The MARS beamline is optimized for X-ray absorption spectroscopy techniques (XANES/EXAFS), powder diffraction (XRD) but x-ray fluorescence (XRF) analysis, High Energy Resolution Fluorescence Detected -XAS (HERFD-XAS), X-ray Emission (XES) and -XAS/XRD are also possible. A description of the beamline as well as its performances are given in a first part. Then some scientific examples of XAS studies from users are presented which cover a wide variety of topics in radiochemistry and nuclear materials.

A comprehensive analysis of X-ray absorption data obtained at the U L 3-edge for a systematic series of single-valence (UO2, KUO3, UO3) and mixed-valence uranium compounds (U4O9, U3O7, U3O8) is reported. High-energy resolution fluorescence detection (HERFD) X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) methods were applied to evaluate U­(IV) and U­(V) environments, and in particular, to investigate the U3O7 local structure. We find that the valence state distribution in mixed-valence uranium compounds cannot be confidently quantified from a principal component analysis of the U L 3-edge XANES data. The spectral line broadening, even when applying the HERFD-XANES method, is sensibly higher (∼3.9 eV) than the observed chemical shifts (∼2.4 eV). Additionally, the white line shape and position are affected not only by the chemical state, but also by crystal field effects, which appear well-resolved in KUO3. The EXAFS of a phase-pure U3O7 sample was assessed based on an average representation of the expanded U60O140 structure. Interatomic U–O distances are found mainly to occur at 2.18 (2), 2.33 (1), and 3.33 (5) Å, and can be seen to correspond to the spatial arrangement of cuboctahedral oxygen clusters. The interatomic distances derived from the EXAFS investigation support a mixed U­(IV)–U­(V) valence character in U3O7.