Two new lithium uranyl tungstates Li 2(UO 2)(WO 4) 2 and Li 2(UO 2) 4(WO 4) 4O with framework based on the uranophane sheet anion topology

Abstract

Two new lithium uranyl tangstates Li 2(UO 2)(WO 4) 2 and Li 2(UO 2) 4(WO 4) 4O have been prepared by high-temperature solid state reactions of Li 2CO 3, U 3O 8 and WO 3. For each compound, the crystal structure was determined by single crystal X-ray diffraction data, using a Brucker diffractometer, equipped with a SMART CCD detector and Mo Kα radiation. The crystal structures were solved at room temperature by direct methods followed by Fourier difference techniques, and refined by a least square procedure on the basis of F 2 for all independent reflections, to R 1=0.035 for 65 refined parameters and 807 reflections with I⩾2 σ( I) for Li 2(UO 2)(WO 4) 2 and to R 1=0.051 for 153 refined parameters and 1766 reflections with I⩾2 σ( I) for Li 2(UO 2) 4(WO 4) 4O. The crystal structure of Li 2(UO 2)(WO 4) 2 is formed by perovskite sheets of WO 6 octahedra, one octahedron thickness, connected together by (UO 5) ∞ infinite chains, and creating tunnels parallel to the c-axis. The lithium atoms are localized in the tunnels. The structure can be deduced from that of U MO 5 ( M=Mo, V, Nb) compounds by the replacement of half U atoms by Li. The crystal structure of Li 2(UO 2) 4(WO 4) 4O consists of UO 7 pentagonal bipyramids, UO 6 tetragonal bipyramids and WO 6 distorted octahedra linked together to form a three-dimensional framework creating paralleled channels filled with lithium cations. The structure can also be described by the stacking of layers with the uranophane sheet anion topology similar to those obtained in U MO 5 ( M=Mo, V, Nb, Sb) compounds with an ordered population of pentagons by U and Li and of squares by U and W. The measured conductivities are comparable to those of the better Li + ion conductor solid electrolytes such as Lisicon or Li- β-alumina. Crystallographic data: Li 2(UO 2)(WO 4) 2, orthorhombic symmetry, space group Pbcn and unit cell parameters a=7.9372(15) Å, b=12.786(2) Å, c=7.4249(14) Å, ρ cal=6.87(2) g/cm 3, ρ mes=6.89(1) g/cm 3 and Z=4. Li 2(UO 2) 4(WO 4) 4O, monoclinic symmetry, space group C2/ c and unit cell parameters a=14.019(4) Å, b=6.3116(17) Å, c=22.296(6) Å, β=98.86(3)°, ρ cal=7.16(2) g/cm 3, ρ mes=7.25(3) g/cm 3 and Z=4.