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 ma y not coordinate with elements to be separated, knowledge of the coord ination polyhedra of the metal ions is important. In this paper, this information was obtained for the solvate [UO2(NP3)(2)(TBP)(2)] (with T BP = tributylphosphate) using X-ray absorption spectroscopy. It includ es the analysis and ab initio calculation of the EXAFS and XANES spect ra corresponding to the model compound [UO2(NO3)2(TIBP)(2)] (with TIBP = tri-iso-butylphosphate) for which the crystal structure is known. T he EXAFS results show little differences between the coordination poly hedra of the uranium ions engaged in both compounds, at low (77 K) or room (295 K) temperatures. The uranium eight-coordination polyhedron i s composed of two uranyl oxygens located at 1.76 Angstrom, four nitrat e (bidentate) oxygens located at 2.51 Angstrom and two phosphate oxyge ns located at 2.42 Angstrom. Finally, investigation of the XANES spect ra fully confirmed these results. (C) 1997 Elsevier Science Ltd.