XAS study of actinide and lanthanide solvent extraction compounds .1. UO2(NO3)(2)(TIBP)(2) and UO2(NO3)(2)(TBP)(2) (with TIBP equals tri-isobutylphosphate and TBP equals tributylphosphate)

For the design of new liquid-liquid extracting agents, which may or may not coordinate with elements to be separated, knowledge of the coordination polyhedra of the metal ions is important. In this paper, this information was obtained for the solvate [UO2(NP3)(2)(TBP)(2)] (with TBP = tributylphosphate) using X-ray absorption spectroscopy. It includes the analysis and ab initio calculation of the EXAFS and XANES spectra corresponding to the model compound [UO2(NO3)2(TIBP)(2)] (with TIBP = tri-iso-butylphosphate) for which the crystal structure is known. The EXAFS results show little differences between the coordination polyhedra of the uranium ions engaged in both compounds, at low (77 K) or room (295 K) temperatures. The uranium eight-coordination polyhedron is composed of two uranyl oxygens located at 1.76 Angstrom, four nitrate (bidentate) oxygens located at 2.51 Angstrom and two phosphate oxygens located at 2.42 Angstrom. Finally, investigation of the XANES spectra fully confirmed these results. (C) 1997 Elsevier Science Ltd.