Relativistic XANES calculations of Pu hydrates

Abstract

A theoretical analysis of x-ray absorption near edge structure (XANES) at ${L}_{3}$ and ${L}_{1}$ edges for Pu hydrates with a formal oxidation state of Pu ranging from +3 to +6, using the ab initio multiple scattering code FEFF7, is presented. For each hydrate our calculations reproduce well the white line intensity and relative peak positions, which are the features commonly used for formal valence identification. In order to achieve such a degree of coincidence between theory and experiment, it was necessary to use a relativistic Dirac-Fock treatment of atomic densities and mixed Dirac-Fock--local-density-approximation exchange-correlation potentials, not considered in previous actinide studies. We find that most of the white line intensity at ${L}_{3}$ edges originates from scattering of the photoelectron, i.e., x-ray absorption fine structure. We also show that the white line shoulder peak, present in the case of plutonyl compounds $({\mathrm{Pu}}^{5+}$ and ${\mathrm{Pu}}^{6+}\mathrm{})$, is due largely to constructive interference of scattering paths containing axial oxygens. Limitations of the FEFF7 code and ways to improve the quantitative agreement between XANES calculations and experimental spectra are also discussed.