Crystal structures of tetravalent uranium fluorides obtained in the presence of hydrazine from uranyl source

Abstract

Four new tetravalent uranium fluorides have been obtained by using hydrothermal route from a hexavalent uranium precursor (UO2(NO3)2·6H2O or UO2(SO4)·4H2O) in the presence of hydrazine and HF in water. The major phase is the hydrated uranium fluoride U3F12(H2O) (1) and in some conditions, four other phases are formed in mixture with the latter. Both compounds 2 and 3 exhibit layered structures with intercalated hydrazinium cations. UF5·H3N-NH2 (2) consists of double uranium fluoride sheets with nine-fold coordinated uranium (UF9, tricapped trigonal prism). It is related to the anionic archetype [UF5]−·X+ or [U2F10]2−·Y2+ (depending of the charge of the counter-cations) previously reported in literature. U3F4(SO4)6·2NH4·H3N-NH3 (3) contains trinuclear μ3-fluoro-centered uranium (UO6F3, tricapped trigonal prism) core linked to each other via the sulfate groups in order to form infinite sheets intercalated by ammonium and diprotonated hydrazinium cationic species. It is a new type of uranium arrangement for tetravalent uranium sulfates. The phases NaU2F9 (4) and NaU6F25 (5) appeared when sodium sulfate is added to the reaction medium. Crystal structure of 4 can be viewed from the connection of double uranium fluorides layers of nine-fold coordinated uranium (UF9) centers, similar to that observed in the compound 2. Due to the small size of sodium cation, the resulting [UF5]− sheets are connected to each other via the remaining non-bonded fluoride anions in order to form a three-dimensional network with channels encapsulating the alkali atoms. The structure of NaU6F25 (5) corresponds to the stacking of double layers of 6-membered ring sub-net of UF9 units, resulting in the formation of elongated cavities in which sodium atoms are located.