New layered structure of Bi2W2O9determined 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.