Synthesis and crystal structure of α and β-Rb6U5V2O23, a new layered compound

A new alkali uranyl vanadate, Rb6U5V2O23 has been synthesized by solid-state reactions. According to synthesis method, two allotropic varieties have been found and their structures determined from single-crystal X-ray diffraction data. Both compounds crystallize in the monoclinic system with space groups C2/c and P2(1)/n for alpha and beta varieties, respectively. alpha-Rb6U5V2O23 unit-cell parameters are a = 24.887(8) Angstrom, b = 7.099(2) Angstrom, c = 14.376(4) Angstrom, beta = 103.92(1)degrees, V = 2465.2(6) Angstrom(3), Z = 4, rho(mes) = 5.86(2) gcm(-3), rho(cal) = 5.85(1) gcm(-3) and for beta-Rb6U5V2O23 a = 7.1635(9) Angstrom, b = 14.079(2) Angstrom, c = 24.965(4) Angstrom, beta = 90.23(1)degrees, V = 2517.8(6) Angstrom(3), Z = 4; rho(mes) = 5.79(2) gcm(-3), rho(cal) 5.73(1) g cm(-3). A full-matrix least-squares refinement yielded, respectively, R = 0.032 and 0.041 for 1307 and 2276 independent reflections with I>2sigma collected on a Smart diffractometer (MoKalpha radiation). Both structures are characterized by [(UO2)(5)(VO4)(2)O-5](6-) layers which are flat and parallel to the (10 (1) over bar) plane in alpha variety, corrugated and parallel to the (010) plane in the beta variety. The layers are built up from VO4 tetrahedra and UO7 pentagonal bipyramids. In beta variety, UO6 distorted octahedra can also be considered. The UO7 pentagonal bipyramids are associated by sharing opposite equatorial edges to form zig-zag infinite chains (UO5)(infinity) parallel to the b-axis in alpha variety and to the a-axis in beta variety, respectively. These chains are linked together on one side by VO4 tetrahedra and on other side in alpha variety by UO7 pentagonal bipyramids sharing corner and opposite edges and in beta variety by UO6 octahedra sharing opposite edges. Both structures differ by several characteristics. In alpha variety, the layers are flat according to the rows Of VO4 tetrahedra parallel to the b-axis whose apical oxygen atoms are alternatively pointing on the both sides in each row whereas in beta variety, they are all pointing in the same side in a row parallel to a and in opposite side in the next, yielding to corrugated layers. (C) 2003 Elsevier Science (USA). All rights reserved.