X-RAY-POWDER AND ELECTRON-DIFFRACTION STUDIES OF THE AVERAGE STRUCTURE AND SUPERSTRUCTURE OF BILA2O4.5

An average structure of BiLa2O4.5 has been determined by X-ray powder diffraction and refined by the Rietveld refinement procedure (R3m space group, a = 3.963(1) Angstrom; c = 9.964(4) Angstrom; hexagonal setting), The fluorite-type structure, with the unit cell stretched along [111] direction and with oxygen atoms displaced slightly, contains statistically distributed Bi/La and O atoms. A hexagonal superstructure unit cell of BiLa2O4.5 (a = 31.67 Angstrom, c = 19.93 Angstrom), probably a consequence of oxygen ordering, has been determined by electron diffraction. A comparison of the six phases found in the Bi2O3-La2O3 system is given to indicate their common structural features. BiLa2O4.5 can be treated as a representative of the Bi(3)RE(5)O(12)-type phases (RE=Y, La-Lu). (C) 1995 Academic Press, Inc.