Synthesis and crystal structure of the pentahydrated uranyl orthovanadate (UO2)(3)(VO4)(2)center dot 5H(2)O, precursor for the new (UO2)(3)(VO4)(2) uranyl-vanadate

The pentahydrated uranyl orthovanadate (UO2)(3)(VO4)(2). 5H(2)O was synthes ized in shaken aqueous solution at 60 degrees C, by reaction of UO3 or a ur anyl salt (acetate or nitrate) and V2O5. Single crystals were obtained by h ydrothermal reaction at 180 degrees C. It crystallizes in an orthorhombic s ystem, space group Cmcm, with a = 17.978(2)Angstrom, b = 13.561(2)Angstrom, c = 7.163(1)Angstrom, V = 1746.3 Angstrom(3), Z = 4, rho(cal) = 4.3 g . cm (-3). The structure has been refined to R = 0.048 and R w = 0.047 using 102 0 independent reflections with I>3 sigma(I) collected on a Philips PW 1100 automated diffractometer using MoK alpha radiation. Uranyl ions have tao di stinct functions. U(1)O-2(2+) uranyl ions are surrounded in an equatorial p lane by five oxygen atoms of four different VO4 tetrahedra which form penta gonal UO7 bipyramids. The linkage of UO7 bipyramids and VO4 tetrahedra lead s to (UVO6)(infinity) layers parallel to the (100) plane. The connection be tween these sheets is achieved by the other U(2)O-2(2+) uranyl ions and wat er molecules. Thus, the compound may be considered a uranyl vanadate of ura nyl and can be formulated (UO2)(0.5)[(UO2)(VO4)]. 2.5H(2)O. Dehydration is reversible and leads to a new anhydrous compound (UO2)(3)(VO4)(2) after int ermediate steps corresponding to different hydrates. Unfortunately, the anh ydrous product cannot be synthesized directly from UO3 and V2O5 by solid-st ate reaction. (C) 2000 Academic Press.