Probing Covalency in the UO3 Polymorphs by U M4 edge HR-XANES

Local atomic and electronic structure investigations of uranium trioxide (UO3) crystalline phases performed by the U M-4 edge HR-XANES technique is presented. The experimental U M4 edge HR-XANES spectra of alpha-UO3, beta-UO3 and gamma-UO3 polymorphic phases are compared with spectra of uranate (CaU2O7) and uranyl (UO3 center dot 1-2(H2O)) compounds. We describe a finger print approach valuable for characterization of variations of U-O axial bond lengths. Theoretical calculations of spectra using full-multiple-scattering theory (FEFF9.6 code) are performed. We have tested and selected input parameters, which provide best agreement between experimental and calculated spectra.