THE CRYSTAL-STRUCTURE OF RUSSELLITE - A REDETERMINATION USING NEUTRON POWDER DIFFRACTION OF SYNTHETIC BI2WO6

The crystal structure of the displacive ferroelectric mineral russellite, Bi2WO6, has been determined using Rietveld profile refinement of high-resolution, time of flight, neutron powder diffraction data on the synthetic compound. Russellite is orthorhombic, Pca2(1), with a 5.43726(2) angstrom, b 16.43018(5) angstrom, c 5.458422) angstrom, Z = 4 and is isostructural with the bismuth molybdate mineral koechlinite, Bi2MoO6. The structure consists of layers of tilted WO6 octahedra sandwiched between layers of bismuth and oxygen with the tungsten displaced from the centre of the octahedron by 0.278 angstrom. The orientations of the lone-pair electrons in the Bi3+ cations have been inferred from the 3.0 angstrom coordination shells of both crystallographically independent bismuths, and have been found to be non-centrosymmetric, an effect which may give rise to the tilting of the WO6 octahedra. New laboratory source X-ray powder diffraction data are presented for russellite, which, with supplementary synchrotron powder diffractometry, corroborate the new space group and structure determination.