Synthesis, crystal structure and electrical characterization of Cs4[(UO2)2(V2O7)O2], a uranyl divanadate with chains of corner-sharing uranyl square bipyramids

A new cesium uranyl vanadate Cs-4[(UO2)2(V2O7)02] has been synthesized by solid-state reaction and its structure determined from single-crystal X-ray diffraction data. It crystallizes in the orthorhombic symmetry with space group Pmmn and following cell parameters: a = 8.4828(15) Angstrom, b = 13.426(2) Angstrom and c = 7.1366(13) Angstrom, V - 812.8(3) Angstrom(3), Z = 2 with rho(mes) = 5.39(2) g/cm(3) and rho(cal) = 5.38(1) g/cm(3). A full-matrix least-squares refinement on the basis of F2 yielded R-1 = 0.027 and wR(2) = 0.066 for 62 parameters with 636 independent reflections with I greater than or equal to 2sigma (I) collected on a BRUKER AXS diffractometer with MoKalpha radiation and a charge-coupled device detector. The crystal structure is characterized by (2)(infinity)[(UO2)(2)(V2O7)O-2](4-) corrugated layers parallel to (001). The layers are built up from distorted (UO2)O-4 octahedra and divanadate V2O7 units resulting from two VO4 tetrahedra sharing corner. The distorted uranyl octahedra (UO2)O-4 are linked by corners to form infinite (1)(infinity)[UO5](4-) chains parallel to the a-axis. These chains are linked together by symmetrical divanadate units sharing two corners with each chain, the two last corners being oriented towards the same interlayer. The cohesion of the structure is assured by interlayer Cs+ ions. Their mobility within the interlayer space gives rise to a cationic conductivity with an important jump between 635degreesC and 680degreesC. Cs-4[(UO2)(2)(V2O7)O-2] is readily decomposed by water at 60degreesC to form the Cs-carnotite analog Cs-2(UO2)(2)(V2O8) compound. (C) 2003 Elsevier Inc. All rights reserved.