THE CRYSTAL STRUCTURE OF FERROELECTRIC Bi2WO6 AT 961 K

The Aurivillius phase Bi2WO6 has been found to undergo a phase transition at 933 K from space group Pca2(1), to Aba2 in addition to the well characterised first order transition at 1233 K. The structural phase transition can be seen to have occurred via a mechanism of rigid unit rotation of WO6 octahedra around the crystallographic b axis. The crystal structure of the higher temperature phase has been refined from high- resolution neutron time-of-flight powder data using the Rietveld method. At 961 K, Bi2WO6 has lattice constants a = 5.49048(1) angstrom, b = 16.52318(2) angstrom, c = 5.52278(1) angstrom, and Z = 4 and consists of layers of Bi2O22+ and WO42- lying perpendicular to the b axis, with the tungsten atom displaced 0.216 A from the centre of a moderately regular octahedron. The orientation of the lone-pair associated with the bismuth cation has been inferred from the 3.0 angstrom coordination shell. The X-ray data for the highest temperature phase of Bi2WO6 has been re-interpreted in the light of this new structure and a potential structural model for this phase has been proposed.