Local Symmetry Effects in Actinide 4f X‑ray Absorption in Oxides

A systematic X-ray absorption study at actinide N 6,7 (4f → 6d transitions) edges was performed for light-actinide oxides including data obtained for the first time for NpO2, PuO2, and UO3. The measurements were supported by ab initio calculations based on local-density-approximation with added 5f–5f Coulomb interaction (LDA+U). Improved energy resolution compared to common experiments at actinide L 2,3 (2p → 6d transitions) edges allowed us to resolve the major structures of the unoccupied 6d density of states (DOS) and estimate the crystal-field splittings in the 6d shell directly from the spectra of light-actinide dioxides. The measurements demonstrated an enhanced sensitivity of the N 6,7 spectral shape to changes in the compound crystal structure. For nonstoichiometric NpO2–x , the filling of the entire band gap with Np 6d states was observed thus supporting a phase coexistence of Np metal and stoichiometric NpO2 which is in agreement with the tentative Np–O phase diagram.