SUPERSTRUCTURES IN THE VANADIUM CONTAINING BISMUTH OXIDES. S. Kashida, A. Kuwasawa and T. Saito, Faculty of Science, Niigata University, Ikarashi, Niigata, 950-21, Japan

Bismuth sesquioxide has attracted wide interest as fuel cell materials. It is an excellent oxygen ion conductor in its high temperature phase. This high temperature fluorite phase is apparently stabilized to room temperature, by doping transition metal oxides. Recent electron diffraction and high resolution electron microscopic studies have disclosed a variety of phases in these 'stabilized bismuth oxides' [W. Zhou, J. Solid State Chem, 76, 290 (1988)].

Using single crystal X-ray diffraction method, we have investigated the structure in the Bi₂O₃-V₂O₅ system. This system shows a sequence of cation ordering phases. Vanadium atoms occupy the cation sites, starting from each at far neighbor sites, since the vanadium-vanadium interaction is of Coulombic and will be repulsive. The 9Bi₂O₃-V₂O₅ compound has a 3x3x3 superstructure derived from the fluorite cell. Vanadium atoms are located at every third layer along the [111] axis. In the layer they occupy the third neighbor cation sites. Oxygen vacancies are concentrated at nearest neighbor sites of vanadium ions.

The 6Bi₂O₃-V₂O₅ compound has a pseudo monoclinic structure. The lattice is represented using the fluorite cell as, a=3/2(1,1,2), b=3/2(-1,1,0), c=1/2(-5,-5,2) and beta=107.13. The structure is composed of six (111) layers stacked along the c-axis, each containing 18 metal atoms. The cation arrangement can be expressed by the number of vanadium atoms as 4-0-4-4-0-4. The lattice has square wave type displacements (0.6Å) along the b-axis. The large displacements are attributed to the electrostatic repulsion between vanadium atoms in the nearest neighbor cation sites. A long period trilling structure with 21 (111) layers was observed around the composition Bi₂O₃:V₂O₅=9:1. A microscopic model is presented for this trilling structure, where vanadium atoms come to the nearest neighbor sites at every seven layer and share oxygen vacancies in common.