X-RAY STUDY OF THE CATION DISTRIBUTION IN THE TERNARY OXIDE, 6 BI2O3-V2O5

The structure of the ternary oxide 6 Bi2O3-V2OS has been investigated using single crystal X-ray diffraction data. It has a fluorite-tyge su perstructure with a pseudo monoclinic cell, a = 20.023 Angstrom, b = 1 1.668 Angstrom, c = 20.472 Angstrom, and beta = 107.13 degrees, (the r elation to the fluorite subcell is a similar or equal to 3/2[1, 1, 2]( c), b similar or equal to 3/2[(1) over bar, 1, 0](c), and c similar or equal to 1/2[(5) over bar, (5) over bar, 2](c)), but the real crystal symmetry is P (1) over bar. The positions of metal atoms are refined to R = 0.12. The structure is characterized by six cation layers along the c axis, each containing 18 metal atoms. The cation arrangement ca n be represented using the numbers of V atoms as 4-0-4-4-0-4. Along th e b axis, the crystal has square wave type displacements (similar or e qual to 0.6 Angstrom). The origin of this large distortion is attribut ed to the Coulomb repulsion between V atoms in the nearest neighbor si tes. (C) 1996 Academic Press, Inc.