Synthesis and crystal structure of new uranyl tungstates M2(UO 2)(W 2O 8) ( M=Na, K), M2(UO 2) 2(WO 5)O ( M=K, Rb), and Na 10(UO 2) 8(W 5O 20)O 8

Abstract

The solid-state reactions of UO 3 and WO 3 with M 2CO 3 ( M=Na, K, Rb) at 650°C for 5 days result, accordingly the starting stoichiometry, in the formation of M 2(UO 2)(W 2O 8) ( M=Na ( 1), K ( 2)), M 2(UO 2) 2(WO 5)O ( M=K ( 3), Rb ( 4)), and Na 10(UO 2) 8(W 5O 20)O 8 ( 5). The crystal structures of compounds 2, 3, 4, and 5 have been determined by single-crystal X-ray diffraction using Mo( Kα) radiation and a charge-coupled device detector. The crystal structures were solved by direct methods and Fourier difference techniques, and refined by a least-squares method on the basis of F 2 for all unique reflections. For ( 1), unit-cell parameters were determined from powder X-ray diffraction data. Crystallographic data: 1, monoclinic, a=12.736(4) Å, b=7.531(3) Å, c=8.493(3) Å, β=93.96(2)°, ρ cal=6.62(2) g/cm 3, ρ mes=6.64(1) g/cm 3, Z=4; 2, orthorhombic, space group Pmcn, a=7.5884(16) Å, b=8.6157(18) Å, c=13.946(3) Å, ρ cal=6.15(2) g/cm 3, ρ mes=6.22(1) g/cm 3, Z=8, R1=0.029 for 80 parameters with 1069 independent reflections; 3, monoclinic, space group P2 1/ n, a=8.083(4) Å, b=28.724(5) Å, c=9.012(4) Å, β=102.14(1)°, ρ cal=5.83(2) g/cm 3, ρ mes=5.90(2) g/cm 3, Z=8, R1=0.037 for 171 parameters with 1471 reflections; 4, monoclinic, space group P2 1/ n, a=8.234(1) Å, b=28.740(3) Å, c=9.378(1) Å, β=104.59(1)°, ρ cal=6.13(2) g/cm 3, ρ mes =6.19(3) g/cm 3, Z=8, R1=0.037 for 171 parameters with 1452 reflections; 5, monoclinic, space group C2/ c, a=24.359(5) Å, b=23.506(5) Å, c=6.8068(14) Å, β=94.85(3)°, ρ cal=6.42(2) g/cm 3, ρ mes =6.39(3) g/cm 3, Z=8, R1=0.036 for 306 parameters with 5190 independent reflections. The crystal structure of 2 contains linear one-dimensional chains formed from edge-sharing UO 7 pentagonal bipyramids connected by two octahedra wide (W 2O 8) ribbons formed from two edge-sharing WO 6 octahedra connected together by corners. This arrangement leads to [UW 2O 10] 2− corrugated layers parallel to (001). Owing to the unit-cell parameters, compound 1 probably contains similar sheets parallel to (100). Compounds 3 and 4 are isostructural and the structure consists of bi-dimensional networks built from the edge- and corner-sharing UO 7 pentagonal bipyramids. This arrangement creates square sites occupied by W atoms, a fifth oxygen atom completes the coordination of W atoms to form WO 5 distorted square pyramids. The interspaces between the resulting [U 2WO 10] 2− layers parallel to (10 1 ̄ ) plane are occupied by K or Rb atoms. The crystal structure of compound 5 is particularly original. It is based upon layers formed from UO 7 pentagonal bipyramids and two edge-shared octahedra units, W 2O 10, by the sharing of edges and corners. Two successive layers stacked along the [100] direction are pillared by WO 4 tetrahedra resulting in sheets of double layers. The sheets are separated by Na + ions. The other Na + ions occupy the rectangular tunnels created within the sheets. In fact complex anions W 5O 20 10− are built by the sharing of the four corners of a WO 4 tetrahedron with two W 2O 10 dimmers, so, the formula of compound 5 can be written Na 10(UO 2) 8(W 5O 20)O 8.