Ammonium Polyuranates: Old Dog, New Structural Tricks.

Abstract

The crystal structure of ammonium polyuranates xUO3·yNH3·zH2O has been investigated. Powder X-ray diffraction (PXRD) has been employed to define single-phase samples within a series of synthesized compounds, which are further characterized by elemental analysis to ascertain the stoichiometry, revealing compositions of 3UO3·NH3·5H2O and 2UO3·NH3·3H2O. Analysis using extended X-ray absorption fine structure and vibrational spectroscopy has elucidated that both 3UO3·NH3·5H2O and 2UO3·NH3·3H2O possess a local structure similar to the metaschoepite─layered U(VI) oxohydroxide UO3·2H2O, but with H2O and NH4+ groups in the interlayers. The structures of ammonium polyuranates are solved from PXRD data, revealing their relationship to the U(VI) oxohydroxide with the established composition of NH4[(UO2)3O2(OH)3]·3H2O and NH4[(UO2)2O2(OH)]·2H2O for 3UO3·NH3·5H2O and 2UO3·NH3·H2O, respectively. These structures maintain the arrangement of U-O polyhedra as pentagonal bipyramids. However, disparities in lattice parameters, space group, and layer topology from UO3·2H2O emphasize significant structural modifications resulting from the substitution of water by ammonium. Moreover, the anion topology of the NH4[(UO2)2O2(OH)]·2H2O has no analogues among uranium oxohydroxide minerals. Notably, ammonium polyuranates, when compared, have minimal alterations in lattice parameters regardless of the presence of ammonia within the structure. The revealed results contribute valuable insights into the UO3-NH3-H2O system and hold potential applications in the field of nuclear power as ammonium polyuranates form during actinide precipitation in back-end of the nuclear fuel cycle and also serve as precursors in the fabrication of nuclear fuel.