An electron diffraction and XRD study of the alpha(1)' and alpha(1)'' fluorite-related phases in the (1-x)1/2Bi(2)O(3)center dot xCaO, 0.20 < x < 0.29, system

Two fluorite-related modulated phases (alpha(1)' and alpha(1)") occurring i n the (1-x)1/2Bi(2)O(3). xCaO, 0.20 < x < 0.29, system have been carefully investigated via electron diffraction and XRD. The satellite reflections ac companying the strong fluorite-type substructure Bragg reflections were alw ays found to be of G(F)+/- similar to 1/8[224](F)* (F for fluorite)-type an d to be consistent with a lowering of the cubic m3m point group symmetry of fluorite to rhombohedral (3) over bar m point group symmetry in both cases . The type of rhombohedral symmetry, however, was found to be quite distinc t. The a; phase is shown to be a two-dimensional incommensurately modulated phase of R (3) over bar m (epsilon 00) 11 superspace symmetry while the al pha(1)" phase is shown to be a rhombohedral, commensurately modulated, supe rstructure phase. Ca/Bi metal ion ordering on the essentially ccp cation ar ray is shown to be largely responsible for the observed satellite reflectio ns and an expression for this ordering is derived using a modulation wave a pproach to structural characterization. (C) 2000 Academic Press.