X-Ray Absorption Sensitivity at 5 d Edges of High-Valent Light Actinides to Crystal-Field Strength and Covalency...

The $5f$ states were probed in ${\mathrm{ThO}}_2$ and U(VI) and U(V) oxides using x-ray absorption spectroscopy at the actinide $O_{4,5}$ edges. Measured data were analyzed by several approaches, including atomic, crystal-field theory and the Anderson impurity model, to take into account the hybridization of actinide valence states with oxygen $2p$ states. For Th(IV), the $5f$ states are mainly affected by the crystal-field interaction with the closest neighbors, as can be seen from the corresponding spectrum of ${\mathrm{ThO}}_2$ . In turn, for U(VI) and U(V) oxides, the U $5f$ -O $2p$ charge transfer and high degree of covalency in the chemical bonding play a decisive role increasing the $5f$ occupancy and consequently governing the ground and excited state properties. That is additionally illustrated by results from the calculations of the U $5d$ x-ray photoemission spectra for U(V) in the lattice of La-doped ${\mathrm{UO}}_2$ .