Synthesis and Crystal Structure of the Pentahydrated Uranyl Orthovanadate (UO2)3 (VO4)2·5H2O, Precursor for the New (UO2)3(VO4)2 Uranyl-Vanadate

The pentahydrated uranyl orthovanadate (UO2)3(VO4)2·5H2O was synthesized in shaken aqueous solution at 60°C, by reaction of UO3 or a uranyl salt (acetate or nitrate) and V2O5. Single crystals were obtained by hydrothermal reaction at 180°C. It crystallizes in an orthorhombic system, space group Cmcm, with a=17.978(2) ?, b=13.561(2) ?, c=7.163 (1) ?, V=1746.3 ?3, Z=4, rhocal=4.3 g·cm-3. The structure has been refined to R=0.048 and Rw=0.047 using 1020 independent reflections with I>3sigma(I) collected on a Philips PW 1100 automated diffractometer using MoKalpha radiation. Uranyl ions have two distinct functions. U(1)O2+2 uranyl ions are surrounded in an equatorial plane by five oxygen atoms of four different VO4 tetrahedra which form pentagonal UO7 bipyramids. The linkage of UO7 bipyramids and VO4 tetrahedra leads to (UVO6)infinity layers parallel to the (100) plane. The connection between these sheets is achieved by the other U(2)O2+2 uranyl ions and water molecules. Thus, the compound may be considered a uranyl vanadate of uranyl and can be formulated (UO2)0.5[(UO2)(VO4)]·2.5H2O. Dehydration is reversible and leads to a new anhydrous compound (UO2)3(VO4)2 after intermediate steps corresponding to different hydrates. Unfortunately, the anhydrous product cannot be synthesized directly from UO3 and V2O5 by solid-state reaction.