Delocalized versus localized unoccupied 5f states and the uranium site structure in uranium oxides and glasses probed by x-ray-absorption near-edge structure

X-ray-absorption near-edge structure (XANES) spectroscopy at the M/sub 3,4,5/ and L/sub 3/ edges of uranium and thorium using synchrotron radiation has been used to probe the unoccupied 5f electronic states and local structure of uranium sites in oxides and glasses. The uranium sites in hyperstoichiometric UO/sub 2+//sub x/ (x--0.25, x--0.66) oxides have been studied. The multiple-scattering resonance in the direction of the linear uranyl group UO/sub 2/ /sup 2+/ is identified in the L/sub 3/ XANES spectra of uranyl nitrate hexahydrate. The localization of unoccupied 5f states in uranium and thorium oxides and glasses has been probed by M/sub 4/- and M/sub 5/-edge spectra. The presence of narrow 5f localized unoccupied states in Th- and U-containing glasses is indicated by the symmetric sharp white line due to 3d5f/sup n//sup +1/ final states. The variable width of the white line indicates that the bandwidth of unoccupied 5f states increases by 4 eV going from glasses to oxides. The presence of 5f components in the U 6d conduction band of oxides, due to hybridization between the U 6d,7s and U5f states is shown by a long asymmetric tail of the white line at its high-energy side extending up to 12 eV. No evidence of many-body effects in XANES spectra, due to the configuration interaction between localized 5f configurations which was observed in mixed-valence rare-earth-metal compounds, has been found in Th and U oxides.