Crystal structure of K2V8O21 and Tl2V8O21

The crystal structures of K2V8O21 and Tl2V8O21 have been determined and refined, using both neutron and X-ray powder diffraction data. K2V8O21 (TI2V8O21) crystallises in the monoclinic space group C2/m (C2/m) with the unit cell parameters a = 14.9402(2) (15.1405(4)) Angstrom, b = 3.61823(4) (3.61309(6)) Angstrom, c = 14.7827(2) (14.9712(2)) Angstrom and beta = 91.072(1)degrees (89.967(2)degrees). The structures consist of VO6 octahedra and VO5 pyramids forming [V8O21](2-) units running along the b axis. These units can then be divided into three columns: (i) two edge-sbaring VO5 pyramids, (ii) columns of rock-salt type structure and (iii) single chains of VO5 pyramids. The KM ions are found in tunnels formed by the [V8O2](2-) units. (C) 2004 Elsevier SAS. All rights reserved.