Ammonium Polyuranates: Old Dog, New Structural Tricks.

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.

Uranium oxides structural transformation in human body liquids.

Uranium oxide microparticles ingestion is one of the potential sources of internal radiation doses to the humans at accidental or undesirable releases of radioactive materials. It is important to predict the obtained dose and possible biological effect of these microparticles by studying uranium oxides transformations in case of their ingestion or inhalation. Using a combination of methods, a complex examination of structural changes of uranium oxides in the range from UO2 to U4O9, U3O8 and UO3 as well as before and after exposure of uranium oxides in simulated biological fluids: gastro-intestinal and lung-was carried out. Oxides were thoroughly characterized by Raman and XAFS spectroscopy. It was determined that the duration of expose has more influence on all oxides transformations. The greatest changes occurred in U4O9, that transformed into U4O9-y. UO2.05 and U3O8 structures became more ordered and UO3 did not undergo significant transformation.

From EXAFS of reference compounds to U(VI) speciation in contaminated environments.

Understanding the speciation of technogenic uranium in natural systems is crucial for estimating U migration and bioavailability and for developing remediation strategies for contaminated territories. Reference EXAFS data of model laboratory-prepared uranium compounds (`standards') are necessary to analyze U-contaminated samples from nuclear legacy sites. To minimize errors associated with measurements on different synchrotrons, it is important not only to compare data obtained on environmentally contaminated samples with the literature but also with `standards' collected at the same beamline. Before recording the EXAFS spectra, all reference compounds were thoroughly characterized by Raman spectroscopy and powder X-ray diffraction. The U(VI) local molecular environments in the reference compounds, i.e. uranyl oxyhydroxides, phosphates, carbonates and uranates, were examined using XAFS. Based on the EXAFS fitting results obtained, including the nature of the bonding, interatomic distances and coordination numbers, parameters that are typical for a particular U compound were differentiated. Using data for `standards', U speciation in the sample of radioactively contaminated soil was determined to be a mixture of U oxyhydroxide and carbonate phases.