Crystal-Field and Covalency Effects in Uranates: An X-ray Spectroscopic Study

The electronic structure of U-V- and U-VI-containing uranates NaUO3 and Pb3UO6 was studied by using an advanced technique, namely X-ray absorption spectroscopy (XAS) in high-energy-resolution fluorescence-detection (HERFD) mode. Due to a significant reduction in core-hole lifetime broadening, the crystal-field splittings of the 5f shell were probed directly in HERFD-XAS spectra collected at the U3d edge, which is not possible by using conventional XAS. In addition, the charge-transfer satellites that result from U5f-O2p hybridization were clearly resolved. The crystal-field parameters, 5f occupancy, and degree of covalency of the chemical bonding in these uranates were estimated by using the Anderson impurity model by calculating the U3d HERFD-XAS, conventional XAS, core-to-core (U 4f-3d transitions) resonant inelastic X-ray scattering (RIXS), and U4f Xray photoelectron spectra. The crystal field was found to be strong in these systems and the 5f occupancy was determined to be 1.32 and 0.84 electrons in the ground state for NaUO3 and Pb3UO6, respectively, which indicates a significant covalent character for these compounds.