A new uranyl niobate sheet in the cesium uranyl niobate Cs 9[(UO 2) 8O 4(NbO 5)(Nb 2O 8) 2

Abstract

A new cesium uranyl niobate, Cs 9[(UO 2) 8O 4(NbO 5)(Nb 2O 8) 2] or Cs 9U 8Nb 5O 41 has been synthesized by high-temperature solid-state reaction, using a mixture of U 3O 8, Cs 2CO 3 and Nb 2O 5. Single crystals were obtained by incongruent melting of a starting mixture with metallic ratio=Cs/U/Nb=1/1/1. The crystal structure of the title compound was determined from single crystal X-ray diffraction data, and solved in the monoclinic system with the following crystallographic data: a=16.729(2) Å, b=14.933(2) Å, c=20.155(2) Å β=110.59(1)°, P2 1/ c space group and Z=4. The crystal structure was refined to agreement factors R 1=0.049 and w R 2=0.089, calculated for 4660 unique observed reflections with I⩾2 σ( I), collected on a BRUKER AXS diffractometer with Mo Kα radiation and a CCD detector. In this structure the UO 7 uranyl pentagonal bipyramids are connected by sharing edges and corners to form a uranyl layer ∞ 2 [ U 8 O 36 ] corresponding to a new anion-sheet topology, and creating triangular, rectangular and square vacant sites. The two last sites are occupied by Nb 2O 8 entities and NbO 5 square pyramids, respectively, to form infinite uranyl niobate sheets ∞ 2 [ ( UO 2 ) 8 O 4 ( NbO 5 ) ( Nb 2 O 8 ) 2 ] 9 - stacking along the [010] direction. The Nb 2O 8 entities result from two edge-shared NbO 5 square pyramids. The Cs + cations are localized between layers and ensured the cohesion of the structure. The cesium cation mobility between the uranyl niobate sheets was studied by electrical measurements. The conductivity obeys the Arrhenius law in all the studied temperature domains. The observed low conductivity values with high activation energy may be explained by the strong connection of the Cs + cations to the infinite uranyl niobate layers and by the high density of these cations in the interlayer space without vacant site. Infrared spectroscopy investigated at room temperature in the frequency range 400–4000 cm −1, showed some characteristic bands of uranyl ion and niobium polyhedra.