The role of neptunium oxidation states and coordination in shaping XANES spectra at the Np L3 absorption edge

Abstract

This paper presents the results of a comprehensive study of neptunium oxides in oxidation states +4 to +7 using X-ray absorption near-edge spectroscopy (XANES) and theoretical modeling. Based on laboratory measurements of the L₃-edge absorption, a systematic edge shift and changes in the spectral characteristics with increasing oxidation state were observed, reflecting changes in the local environment of neptunium atoms. Spectral XANES modeling using the FDMNES code and an analysis of the electron density of states (DOS) allowed us to interpret the contributions of the 6d states of neptunium to the formation of the absorption spectra, as well as their hybridization with the 2p states of oxygen. Particular attention is paid to CsNp^VIIO₄, which exhibits characteristic splitting features on the main absorption edge. The results demonstrate the high information content of XANES, especially in combination with theoretical analysis, for studying the electronic structure of actinide compounds.