PREPARATION AND CRYSTAL-STRUCTURE OF SR6TINB4O18

The crystal structure of newly prepared Sr6TiNb4O18 was determined by single-crystal X-ray diffraction and is described here. This compound crystallizes in space group R3m (No. 160; a = 5.6437(5), c = 41.347(3) Angstrom; Z = 3). The structure consists of infinite perovskite-type slabs, five octahedra in thickness, extending parallel to (111)(perovs kite). All Sr ions are 12-coordinated; Nb5+ and Ti4+ are distributed a mong the octahedral M sites. The cation coordination spheres in and bo rdering the gaps between the perovskite type slabs are highly distorte d with bond valence sums indicative of residual chemical strain: Sr-O bonds are stretched, M-O bonds compressed. Nb5+ preferentially occupie s the distorted octahedral sites that border the gaps; the least amoun t of the higher valent metal is found in octahedral sites embedded in the center of the slabs. Similar crystal-chemical effects were reporte d for isostructural Ba6TiNb4O18 and for Sr5TiNb4O17, also a perovskite -slab-type structure but with slices cut parallel to (110)(perovskite) .