Ammonium Polyuranates: Old Dog, New Structural Tricks

The crystal structure of ammonium polyuranates ··O 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 3UO·NH·5HO and 2UO·NH·3HO. Analysis using extended X-ray absorption fine structure and vibrational spectroscopy has elucidated that both 3UO·NH·5HO and 2UO·NH·3HO possess a local structure similar to the metaschoepite─layered U(VI) oxohydroxide UO·2HO, 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 NH and NH for 3UO·NH·5HO and 2UO·NH·HO, respectively. These structures maintain the arrangement of U-O polyhedra as pentagonal bipyramids. However, disparities in lattice parameters, space group, and layer topology from UO·2HO emphasize significant structural modifications resulting from the substitution of water by ammonium. Moreover, the anion topology of the NH 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.