Multiple-scattering calculations of the uraniumL3-edge x-ray-absorption near-edge structure

A theoretical study of the uranium ${\mathit{L}}_3$-edge x-ray absorption near-edge structure (XANES) is presented for several uranium compounds, including oxides, intermetallics, uranyl fluoride, and α-uranium. Calculations were performed using f e f f6, an ab initio multiple-scattering (MS) code that includes the most important features of current theories. The results, which account for both the fine structure χ and the atomiclike background ${\mathrm{\mu }}_0$ of the absorption coefficient μ, are compared to new and previously measured experimental spectra, reavealing very good agreement for most systems. For several compounds, a more detailed theoretical analysis determined the influence of cluster size and scattering order upon the calculated spectra. Results indicate that MS paths and scattering paths that include rather distant atoms make significant contributions for ${\mathrm{UO}}_2$, whereas XANES for crystals with lower symmetry and density can be modeled using only shorter single-scattering paths. In most cases, assumption of a screened final state in the calculation gives better agreement with experiment than use of an unscreened final state. The successful modeling of spectra for a variety of different uranium compounds, with differing spectral features, indicates that the semirelativistic treatment of XANES used here is adequate even for heavy elements. The well-known resonance, observed experimentally for uranyl (${\mathrm{UO}}_2^{2+}$) compounds ≊15 eV above the white line, is successfully modeled here for the first time, using multiple-scattering paths within the O-U-O axial bonds. Overlapping muffin-tin spheres were required in the calculation, probably as a result of the short uranyl axial bonds.