The A1−xUNbO6−x/2 compounds (x=0, A=Li, Na, K, Cs and x=0.5, A=Rb, Cs): from layered to tunneled structure

Attempts to prepare alkaline metal uranyl niobates of composition A1−xUNbO6−x/2 by high-temperature solid-state reactions of A2CO3, U3O8 and Nb2O5 led to pure compounds for x=0 and A=Li (1), Na (2), K (3), Cs (4) and for x=0.5 and A=Rb (5), Cs (6). Single crystals were grown for 1, 3, 4, 5, 6 and for the mixed Na0.92Cs0.08UNbO6 (7) compound. Crystallographic data: 1, monoclinic, P21/c, a=10.3091(11), b=6.4414(10), c=7.5602(5) Å, β=100.65(1), Z=4, R1=0.054 (wR2=0.107); 3, 5 and 7 orthorhombic, Pnma, Z=8, with a=10.307(2), 10.272(4) and 10.432(3) Å, b=7.588(1), 7.628(3) and 7.681(2) Å, c=13.403(2), 13.451(5) and 13.853(4) Å, R1=0.023, 0.046 and 0.036 (wR2=0.058, 0.0106 and 0.088) for 3, 5 and 7, respectively; 6, orthorhombic, Cmcm, Z=8, and a=13.952(3), b=10.607(2) Å, c=7.748(2) Å, R1=0.044 (wR2=0.117).The crystal structure of 1 is characterized by ∞2[UNbO6]- layers of uranophane sheet anion topology parallel to the (100) plane. These layers are formed by the association by edge-sharing of ∞1[UO5]4- chains of edge-shared UO7 pentagonal bipyramids and ∞1[NbO4]3- chains of corner-shared NbO5 square pyramids alternating along the [010] direction. The Li+ ions are located between two consecutive layers and hold them together; the Li+ ions and two layers constitute a neutral “sandwich” {(UNbO6)−–(Li)22+–(UNbO6)−}. In this unusual structure, the neutral sandwiches are stacked one above another with no formal chemical bonds between the neutral sandwiches.The homeotypic compounds 3, 5, 6, 7 have open-framework structures built from the association by edge-sharing in two directions of parallel ∞1[UO5]4- chains of edge-shared UO7 pentagonal bipyramids and ∞1[Nb2O8]6- ribbons of two edge-shared NbO6 octahedra further linked by corners. In 3, 5 and 7, the mono-dimensional large tunnels created in the [001] direction by this arrangement can be considered as the association by rectangular faces of two columns of triangular face-shared trigonal prisms of uranyl oxygens. In 3 and 7, all the trigonal prisms are occupied by the alkaline metal, in 5, they are half-occupied. In 6, the polyhedral arrangement is more symmetric and the tunnels created in the [010] direction are built of face-sharing cubes of uranyl oxygens totally occupied by the Cs atoms. This last compound well illustrates the structure-directing effect of the conterion.

Occupation of the tunnels of the uranium niobium polyhedra framework by the alkaline metal in (a) KUNbO6, (b) Rb0.5UNbO5.75 and (c) Cs0.5UNbO5.75.Figure optionsDownload as PowerPoint slide