Local Structure Analysis of Uranyl Phosphates and Arsenates using EXAFS Spectroscopy

Local Structure Analysis of Uranyl Phosphates and Arsenates using EXAFS Spectroscopy

C. Hennig, T. Reich, A. Roßberg, H. Funke, M. Rutsch,
G. Geipel, H. Nitsche, G. Bernhard

Forschungszentrum Rossendorf e.V., Institut für Radiochemie, P.F. 510119, D-01314 Dresden

First EXAFS measurements were taken on the new ROssendorf Beam Line (ROBL) [1]. This beamline is built by the Forschungszentrum Rossendorf as collaborating research group (CRG) at the ESRF in Grenoble. ROBL comprises a radiochemistry laboratory of type B and a materials research station. The radiochemistry station is designed for X-ray absorption spectroscopy whereas the materials research station uses X-ray diffraction techniques. The monochromator is equipped with a Si(111) and Si(311) water cooled double-crystal system of fixed-exit type which covers an energy range of 5keV-35keV. Higher harmonics are rejected by two Si/Pt coated mirrors.
Some saxonian granites contain uranium as hydrothermal crystallized uranyl phosphates and arsenates. Uranium and arsenic appear also in seepage waters of mine tailing piles from uranium mines and give stable complexes. Local structure analysis of these compounds is nessesary to understand the environmental weathering and mineralization processes.
The investigated samples are natural Cu[UO2PO4]2*nH2O, Ba[UO2PO4]2*nH2O, Ca[UO2PO4]2*nH2O, Cu[UO2AsO4]2*nH2O and synthetic H[UO2AsO4]*nH2O. The crystal structures are generally built up by stable layers of [UO2]2+ and [PO4]3-or [AsO4]3- units and the charge neutrality is achived by different interlayer cations.
U LIII-edge and As K-edge EXAFS spectra were collected in transmission, Cu K-edge EXAFS spectra were measured with a multichannel Ge fluorescence detector [2]. The measurements were carried out with a sample orientation of 0° and 45° to the beam direction in order to investigate the influence of polarization effects.
We have measured small but significant bond length differences within this structural isotypic group. Deviations in the coordination numbers are obtained as a result of polarization effects and moreover different Debye-Waller factors in connection with structural defects. In conclusion, the uranyl arsenate layer structure is nearly independent from the interlayer cation arrangement. Our investigation demonstrates the possibility for compensation the lost angle information in EXAFS spectroscopy by using the radial distribution functions at various absorption edges.

[1] ESRF Beamline Handbook 1997 pp.145
[2] Bucher, J.J., et al., Rev. Sci. Instrum. 67 (1996) 1