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 (Tl2V8O21) crystallises in the monoclinic space group C2/m(C2/m) with the unit cell parameters a=14.9402(2) (15.1405(4)) Å, b=3.61823(4) (3.61309(6)) Å, c=14.7827(2) (14.9712(2)) Å and β=91.072(1)° (89.967(2)°). 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-sharing VO5 pyramids, (ii) columns of rock-salt type structure and (iii) single chains of VO5 pyramids. The K/Tl ions are found in tunnels formed by the [V8O21]2− units.