Synthesis, crystal structure and electrical characterization of Cs 4[(UO 2) 2(V 2O 7)O 2], a uranyl divanadate with chains of corner-sharing uranyl square bipyramids

Abstract

A new cesium uranyl vanadate Cs 4[(UO 2) 2(V 2O 7)O 2] 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) Å, b=13.426(2) Å and c=7.1366(13) Å, V=812.8(3) Å 3, Z=2 with ρ mes=5.39(2) g/cm 3 and ρ cal=5.38(1) g/cm 3. A full-matrix least-squares refinement on the basis of F 2 yielded R 1=0.027 and w R 2=0.066 for 62 parameters with 636 independent reflections with I⩾2 σ( I) collected on a BRUKER AXS diffractometer with Mo Kα radiation and a charge-coupled device detector. The crystal structure is characterized by ∞ 2[(UO 2) 2(V 2O 7)O 2] 4− corrugated layers parallel to (001). The layers are built up from distorted (UO 2)O 4 octahedra and divanadate V 2O 7 units resulting from two VO 4 tetrahedra sharing corner. The distorted uranyl octahedra (UO 2)O 4 are linked by corners to form infinite ∞ 1[UO 5] 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 635°C and 680°C. Cs 4[(UO 2) 2(V 2O 7)O 2] is readily decomposed by water at 60°C to form the Cs-carnotite analog Cs 2(UO 2) 2(V 2O 8) compound.