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 P21212 (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 (~175 pm) and long (~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.