Influence of the local atomic structure in the X-ray absorption near edge spectroscopy of neptunium oxo ions

Abstract

Experimental L_III X-ray absorption near edge structure (XANES) spectra of the distorted octahedral neptunium oxo ions NpO₂(OH)₄²⁻, NpO₄(OH)₂³⁻, and NpO₆⁶⁻ are interpreted using relativistic full multiple scattering calculations of the X-ray absorption process. In this series of compounds, the neptunium cation exhibits two different oxidation states, VI and VII, with coordination spheres from di- to tetra oxo for the first two compounds. The comparison between calculated XANES spectra using the feff code and experimental ones shows that the main features in the spectra are determined by the local coordination around the actinide metal center. Furthermore, the projected density of electronic states (DOS) calculated from the XANES simulations using the feff code are compared to calculations using ADF code. They are both discussed in terms of molecular orbitals and qualitative evolution of bonding within this series of compounds.