Synthesis and crystal structure of two new uranyl oxychloro-vanadate layered compounds:: M7(UO2)8(VO4)2O8Cl with M = Rb, Cs

Two new alkali uranyl oxychloro vanadates M-7(UO2)(8)(VO4)(2)O8Cl with M = Rb, Cs, have been synthesized by solid-state reactions and their structures determined from single-crystal X-ray diffraction data. They crystallize in the orthorhombic system with space groups Pmcn and Pmmn, respectively. The a and b unit cell parameters are almost identical in both compounds while the c parameter in the Rb compound is doubled: Rb-a = 21.427(5) Angstrom, b = 11.814(3) Angstrom, c = 14.203(3) Angstrom, V = 3595.1(1) Angstrom(3), Z = 4, rho(mes) = 5.93(2) g/cm(3), rho(cal) = 5.82(1) g/cm(3); Cs-a = 21.458(3) Angstrom, b = 11.773(2) Angstrom, c = 7.495(1) Angstrom, V = 1893.6(5) Angstrom(3), Z = 2, rho(mes) = 6.09(2) g/cm(3), rho(cal) = 6.11(1) g/cm(3). A full-matrix least-squares refinement yielded R-1 = 0.0221, wR(2) = 0.0562 for 2675 independent reflections and R-1 = 0.0386, wR(2) = 0.1042 for 2446 independent reflections, for the Rb and Cs compounds, respectively. Data were collected with Mo(Kalpha) radiation and a charge coupled device (CCD) detector of a Bruker diffractometer. Both structures are characterized by [(UO2)(8)(VO4)(2)O8Cl](n)(7n-) layers parallel to the (001) plane. The layers are built up from VO4 tetrahedra, UO7 and UO6Cl pentagonal bipyramids, and UO6 distorded octahedra. The UO7 and UO6Cl pentagonal bipyramids are associated by sharing opposite equatorial edges to form infinite chains (UO5-UO4Cl-UO5)(n) parallel to the a axis. These chains are linked together by VO4 tetrahedra, UO6 octahedra, UO7 corner sharing and UO6Cl, Cl sharing. Both structures differ simply by the symmetry of the layers. The unit cell contains one centrosymmetric layer in the Cs compound, whereas in the two-layer unit cell of the Rb compound, two non-centrosymmetric consecutive layers are related by an inversion center. The layers appear to be held together by the alkali ions. The mobility of the M+ ions within the interlayer space in M-7(UO2)(8)(VO4)ZO(8)Cl and carnotite analog compounds is compared. (C) 2003 Elsevier Science (USA). All rights reserved.