Аннотация:The crystal structure of ammonium polyuranatesxUO3·yNH3·zH2O has been investigated. Powder X-ray diffraction(PXRD) has been employed to define single-phase samples withina series of synthesized compounds, which are further characterizedby elemental analysis to ascertain the stoichiometry, revealingcompositions of 3UO3·NH3·5H2O and 2UO3·NH3·3H2O. Analysisusing extended X-ray absorption fine structure and vibrationalspectroscopy has elucidated that both 3UO3·NH3·5H2O and2UO3·NH3·3H2O possess a local structure similar to themetaschoepitelayered U(VI) oxohydroxide UO3·2H2O, butwith H2O and NH4+ groups in the interlayers. The structures ofammonium polyuranates are solved from PXRD data, revealingtheir relationship to the U(VI) oxohydroxide with the establishedcomposition 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 latticeparameters, space group, and layer topology from UO3·2H2O emphasize significant structural modifications resulting from thesubstitution of water by ammonium. Moreover, the anion topology of the NH4[(UO2)2O2(OH)]·2H2O has no analogues amonguranium oxohydroxide minerals. Notably, ammonium polyuranates, when compared, have minimal alterations in lattice parametersregardless 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 actinideprecipitation in back-end of the nuclear fuel cycle and also serve as precursors in the fabrication of nuclear fuel.