UL_III Polarized XAFS Studies on Ba[UO₂PO₄]₂.8H₂O

U LIII Polarized XAFS studies on Ba[UO2PO4]2·8H2O
C. Hennig 1, M. A. Denecke 1, A. Roßberg 1, G. Zahn 2,
T. Reich 1, H. Nitsche1
1 Forschungszentrum Rossendorf, Institut für Radiochemie,
PF 510119, D-01314 Dresden
2 Technical University of Dresden, Institute of Crystallography
and Solid State Physics, Zellescher Weg 16,
D-01062 Dresden

An experimental and theoretical study of the U LIII -edge polarization dependent XANES and EXAFS is presented for a single crystal of barium uranyl phosphate hydrate. Ba[UO2PO4]2·8H2O is built up by layers of [UO2]2+ and equatorial coordinated [PO4]3- tetrahedra. Layers are bridged by Ba2+, H+ and H3O+ cations giving an overall tetragonal (or nearly tetragonal) symmetry. The orientation of a crystal obtained by cleaving a larger crystal and used for the XAFS measurements was determined by x-ray diffraction. The layered structure of Ba[UO2PO4]2·8H2O exhibits preferred cleavage along {001} and parallel to {100}. Linear uranyl units are aligned parallel to [001]. {100} represents the least squares plane for the equatorial oxygen atoms. XAFS measurements were performed with the polarization vector aligned in the equatorial plane (parallel to [100] and [110]), as well as with the polarization vector oriented perpendicular to [010], with 90°, 60°, 45°, and 30° angles to [001].
Two different explanations of the resonance » 15 eV above the LIII -edge white-line maximum have been discussed. One explanation, based on pressure dependent measurements of UO3, attributed this feature to shake-up processes [1]. The other interpretation stems from ab initio multiple-scattering calculations using the FEFF program package. These calculations identify this feature as a multiple-scattering resonance associated with the axial U-O bonds in various uranium compounds [2], [3]. Our experimental investigations show that the relative intensity of this resonance increases if the polarization vector is aligned along the axial U-O bonds. This observation confirms the multiple-scattering assignment of this feature and is supported by our own theoretical FEFF calculations.
The U LIII -edge EXAFS show a strong polarization dependence, especially evident in the Fourier transforms at distances corresponding to uranyl axial oxygen atoms and equatorial atoms. This dependence strongly influences the effective coordination numbers, Ni, obtained in the data analysis.
The marked characteristics of such polarization-dependent XAFS spectra renders them useful as a unique indicator of uranyl orientation. For example, the specific binding sites and orientation of sorbed uranyl (actinyl) ions onto mineral surfaces can be determined with this method.

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