Superstructure and twinning in the tetragonal tungsten bronze-type phase Nb7W10O47

Needle-shaped crystals of Nb7W10O47, a member of the solid solution series Nb8-nW9+nO47 with n=1, were prepared from the binary oxides with HgCl2 added as a mineralizer, All investigated crystals were twinned, The data set was measured on a fourling, which consists of 73% of an individual twin domain. The orthorhombic structure (a=1225,7(2), b=3663.3(6), c=394.96(6) pm) was refined in the space group P2(1)2(1)2 (No. 18) to a final R value of 5.11%, The composition of this crystal determined from X-ray data is Nb6.7W10.3O47. Nb7W10O47 crystallizes isostructural to Nb8W9O47 in a threefold superstructure of the tetragonal tungsten bronze type. All atomic positions are considerably shifted out of the planes (001) and (002), leading to alternating short (similar to 175 pm) and long (similar to 220 pm) metal-oxygen distances along the short crystallographic c axis, One octahedral site is exclusively occupied by W, the center of the pentagonal bipyramid preferentially by Nb. (C) 2000 academic Press.