Characteristics of chemical bonding of pentavalent uranium in La-doped UO2

The effect of La doping on the electronic structure of U in UO2 was studied using an advanced technique, namely x-ray absorption spectroscopy (XAS) in the high-energy-resolution uorescence- detection (HERFD) mode at the U 3d3=2 (M4) edge. Thanks to a significant reduction of the core-hole lifetime broadening and distinct chemical shifts of the HERFD-XAS lines, the U(V) formation as a result of La doping was identified. The isolated contribution of U(V) in the M4 HERFD-XAS spectrum reveals the so-called charge-transfer satellites due to the U 5f -O 2p hybridization. The analysis of the experimental data within framework of the Andersson impurity model (AIM) indicates a significant change in the character and degree of covalency for the chemical bonding in the U(V) subsystem of UO2 as compared to undoped UO2 which is a Mott-Hubbard system. The results are also supported by AIM calculations of x-ray photoelectron and optical absorption data.