New layered structure of Bi₂W₂O₉ determined by 1 MV high-resolution electron microscopy

The crystal structure of bismuth tungstate, Bi₂W₂O₉, has been examined by 1 MV high-resolution electron microscopy. The symmetry is orthorhombic with lattice parameters a = 5.43, b = 5.41 and c = 23.7 Å and space group Pna2₁. The point group is determined directly from the images in which each cation site is resolved. The structure is similar to that of conventional layered Bi compounds with respect to the Bi₂O₂ sheets normal to the c axis; however, the layer between the sheets is of a deformed ReO₃ type. The strong chemical bonds between the Bi ions and the apex O atoms cause the rotation of the WO₆ octahedron about the c axis. W ions are displaced antiparallel in the WO₃ layer.