Three-dimensional framework of uranium-centered polyhedra with non-intersecting channels in the uranyl oxy-vanadates A2(UO 2) 3(VO 4) 2O ( A=Li, Na)

Abstract

The uranyl vanadates A 2(UO 2) 3(VO 4) 2O ( A=Li, Na) have been synthesized by solid-state reaction and the structure of the Li compound was solved from single-crystal X-ray diffraction. The crystal structure is built from ∞ 1 [ UO 5 ] 4 - chains of edge-shared U(2)O 7 pentagonal bipyramids alternatively parallel to a ⇒ - and b ⇒ -axis and further connected together to form a three-dimensional (3-D) arrangement. The perpendicular chains are hung on both sides of a sheet ∞ 2 [ ( UO 2 ) ( VO 4 ) 2 ] 4 - parallel to (001), formed by U(1)O 6 square bipyramids connected by VO 4 tetrahedra, and derived from the autunite-type sheet. The resulting 3-D framework creates non-intersecting channels running down the a ⇒ - and b ⇒ -axis formed by empty face-shared oxygen octahedra, the Li + ions are displaced from the center of the channels and occupy the middle of one edge of the common face. The peculiar position of the Li + ion together with the full occupancy explain the low conductivity of Li 2(UO 2) 3(VO 4) 2O compared with that of Na(UO 2) 4(VO 4) 3 containing the same type of channels half occupied by Na + ions in the octahedral sites. Crystallographic data for Li 2(UO 2) 3(VO 4) 2O: tetragonal, space group I41/ amd, a = 7.3303 ( 5 ) Å , c = 24.653 ( 3 ) Å , V = 1324.7 ( 2 ) Å 3 , Z = 4 , ρ mes=5.32(2) g/cm 3, ρ cal=5.36(3) g/cm 3, full-matrix least-squares refinement basis on F 2 yielded, R 1 = 0.032 , w R 2 = 0.085 for 37 refined parameters with 364 independent reflections with I ⩾ 2 σ ( I ) .