Uranium-Induced Changes in Crystal-Field and Covalency Effects of Th⁴⁺ in Th_1-xU_xO₂ Mixed Oxides Probed by High-Resolution X-ray Absorption Spectroscopy

Knowledge of the local Th structure is a prerequisite for a better understanding of the physicochemical properties of the thorium-based mixed oxides (Th-MOX) involved in the Th-based nuclear fuel cycle. The crystalline electric field (CEF) splitting of the 6d shell in Th_1-xU_xO₂ (x = 0.25, 0.5, 0.75) solid solution was probed by the Th L₃ edge high-energy-resolution fluorescence-detected (HERFD) X-ray absorption near-edge spectroscopy (XANES) collected at the L_β5 emission line, which cannot be obtained by conventional X-ray absorption methods. The detected CEF split between the 6d e_g and t_2g orbitals in ThO₂ consisting of ordered Th-O₈ cubes with cubic symmetry is ∼3.5 eV for the Th⁴⁺ ion. Because the split peaks of the white line corresponding to the crystal-field splitting of the unoccupied 6d states were resolved in the HERFD-XANES spectra, the analysis of these split peaks combined with first-principles calculations revealed that an increase of the U content involves the distortion of the Th-O₈ cubes in the Th_1-xU_xO₂ mixed oxides. The lower symmetry of the Th-O₈ cube induced by the incorporated U tends to reduce the local crystal-field around Th⁴⁺ as well as the hybridization of Th 6t_2g-O 2p which is mainly responsible for the covalent property of the Th-O bond. The phenomenon is noticeable in Th_0.25U_0.75O₂, whose CEF splitting is decreased by approximately 10%, and covalent mixing between Th 6d t_2g and O 2p orbitals is substantially reduced compared to that of pure ThO₂.