LOW-TEMPERATURE CRYSTAL-STRUCTURES AND THERMAL-DECOMPOSITION OF URANYL HYDROGEN SELENITE MONOHYDRATE, UO2(HSEO3)(2) H2O AND DIAMMONIUM URANYL SELENITE HEMIHYDRATE, (NH4)(2)UO2(SEO3)(2)CENTER-DOT-0.5H(2)O

The structures of the uranyl hydrogen selenite monohydrate, UO2(HSeO3) (2) . H2O and diammonium uranyl selenite hemihydrate, (NH4)(2)UO2(SeO3 )(2) . 0.5H(2)O have been determined by S-ray diffraction at 193 K and their thermal decomposition studied. The structure of (NH4)(2)UO2(SeO 3)(2) . 0.5H(2)O is layered. The layers are formed by the two unequal selenite ions acting as bridging ligands between three and two U-VI io ns, respectively. The ammonium ions and the water molecule form an arr ay, which through a complex hydrogen bond network holds the layers tog ether. In UO2(HSeO3)(2) . H2O the hydrogen selenite ion acts as a brid ging ligand between two consecutive U-VI ions, leading to a structure of one-dimensional zigzag chains. (NH4)(2)UO2(SeO3)(2) . 0.5H(2)O deco mposes in air between 570 and 640 K in a single thermogravimetric step to a phase containing uranyl selenite with additional selenium dioxid e. The final product, U3O8, is formed between 870 and 930 K. After deh ydration UO2(HSeO3)(2) . H2O decomposes to stoichiometric UO2SeO3 . Se O2. This compound decomposes further to the same non-stoichiometric ur anyl selenite-selenium dioxide phase as the ammonium compound.