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 adde d as a mineralizer, All investigated crystals were twinned, The data set wa s measured on a fourling, which consists of 73% of an individual twin domai n. The orthorhombic structure (a=1225,7(2), b=3663.3(6), c=394.96(6) pm) wa s 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 Nb6.7W10.3 O47. Nb7W10O47 crystallizes isostructural to Nb8W9O47 in a threefold supers tructure of the tetragonal tungsten bronze type. All atomic positions are c onsiderably shifted out of the planes (001) and (002), leading to alternati ng short (similar to 175 pm) and long (similar to 220 pm) metal-oxygen dist ances along the short crystallographic c axis, One octahedral site is exclu sively occupied by W, the center of the pentagonal bipyramid preferentially by Nb. (C) 2000 academic Press.