Local Structure in U(IV) and U(V) Environments: The Case of UO.

A comprehensive analysis of X-ray absorption data obtained at the U -edge for a systematic series of single-valence (UO, KUO, UO) and mixed-valence uranium compounds (UO, UO, UO) 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 UO 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 -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 KUO. The EXAFS of a phase-pure UO sample was assessed based on an average representation of the expanded UO 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 UO.