Ammonium Polyuranates: Old Dog, New Structural Tricks

The crystal structure of ammonium polyuranates xUO 3 ·yNH 3 ·zH 2 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 3 ·NH 3 ·5H 2 O and 2UO 3 ·NH 3 ·3H 2 O. Analysis using extended X-ray absorption fine structure and vibrational spectroscopy has elucidated that both 3UO 3 ·NH 3 ·5H 2 O and 2UO 3 ·NH 3 ·3H 2 O possess a local structure similar to the metaschoepitelayered U­(VI) oxohydroxide UO 3 ·2H 2 O, 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 3UO 3 ·NH 3 ·5H 2 O and 2UO 3 ·NH 3 ·H 2 O, 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 3 ·2H 2 O 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.