Synthesis, crystal structure, cationic mobility, thermal evolution and spectroscopic study of Cs 8(UO 2) 4(WO 4) 4(WO 5) 2 containing infinite uranyl tungstate chains

Abstract

The powder samples and single crystals of the cesium uranyl tungstate compound Cs 8(UO 2) 4(WO 4) 4(WO 5) 2 have been synthesized by high temperature solid state reaction and its structure determined from single crystal X-ray diffraction data. The cesium mobility and vibration modes of uranyl and tungstate polyhedrons have been evidenced using pulverulent samples. It crystallizes in the monoclinic symmetry with space group P2 1/ n and following cell parameters, a = 11.2460(3) Å, b = 13.8113(3) Å, c = 25.7287(3) Å, β = 90.00(3)°, V = 3996.23(17) Å 3 and Z = 4 with ρ mes = 6.079(2) g/cm 3 and ρ cal = 6.087(2) g/cm 3. A full-matrix least-squares refinement on the basis of F 2 yielded R 1 = 0.0379 and wR 2 = 0.0624 for 471 parameters with 14,278 independent reflections with I ≥ 2 σ( I) collected on a Bruker X8 CCD 4K diffractometer with Mo Kα radiation. In this structure, the uranium atoms adopt UO 7 pentagonal bipyramid coordination, while tungsten atoms are in two different environments, WO 4 tetrahedral and WO 5 square pyramidal coordinations. The association of uranyl ions (UO 7) and tungstate oxoanions WO 4 and WO 5, gives infinite chains [ ( UO 2 ) 4 ∞ 1 ( WO 4 ) 4 ( WO 5 ) 2 ] 8 − parallel to [100]. These types of chains has not been previously observed. The association of these chains in the (010) plane gives undulated pseudo-layers stacked along [010]. The cohesion between the chains is assured by alkaline Cs + cations. The conductivity measurements, between 200 and 600 °C, show an Arrhenius law evolution. Infrared spectroscopy investigated at room temperature in the 400–4000 cm −1 wave number range, has allowed the identification of the various modes of vibrations of uranyl and tungstate polyhedrons.