Uranyl silicate nanotubules in Rb${}_2$[(UO${}_2$)${}_2$O(Si${}_3$O${}_8$)]. Synthesis and crystal structure

A new rubidium uranyl silicate, Rb${}_2$(UO${}_2$)${}_2$O(Si${}_3$O${}_8$) (1), was obtained using high-temperature approach from the melt in silica tubes. Its crystal structure was solved by direct methods: hexagonal, P6/m, a = 27.7992(7), c = 7.2346(2) Å, V = 4841.8(3) Å${}^3$, R${}_1$ = 0.033. The structure of 1 represents a new structure type with unprecedented topology not observed before among U(VI) oxides and oxysalts. It is comprised of layers with large voids derived from the U${}_3$O${}_8$ structure formed exclusively by pentagonal UrO${}_5$ bipyramids. The low-occupied Rb sites are located in the interlayer space. The SiO${}_4$ silicate tetrahedra in the structure of 1 share vertices to form rolled [Si${}_6$O${}_{16}$]${}^{8-}$ chains. The nanotubules [(UO${}_2$)(Si${}_6$O${}_{16}$)]${}^{6-}$ penetrate through both U${}_3$O${}_8$-derived layers and Rb interlayer. These tubules are attached to the U${}_3$O${}_8$ derived sheets via uranyl-uranyl interactions and edge-sharing between silicate tetrahedra and UrO${}_5$ bipyramids.