Three-dimensional framework of uranium-centered polyhedra with non-intersecting channels in the uranyl oxy-vanadates A2(UO2)3(VO4)2O (A=Li, Na)

The uranyl vanadates A2(UO2)3(VO4)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 image chains of edge-shared U(2)O7 pentagonal bipyramids alternatively parallel to image- and image-axis and further connected together to form a three-dimensional (3-D) arrangement. The perpendicular chains are hung on both sides of a sheet image parallel to (001), formed by U(1)O6 square bipyramids connected by VO4 tetrahedra, and derived from the autunite-type sheet. The resulting 3-D framework creates non-intersecting channels running down the image- and image-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 Li2(UO2)3(VO4)2O compared with that of Na(UO2)4(VO4)3 containing the same type of channels half occupied by Na+ ions in the octahedral sites.