An Electron Diffraction and XRD Study of the ALPHA1 and ALPHA1 Fluorite-Related Phases in the (1-x) Bi2O3· xCaO, 0.20

Two fluorite-related modulated phases (ʹ1 and ʹʹ1) occurring in the (1-x) Bi2O3·xCaO, 0.20*F (F for fluorite)-type and to be consistent with a lowering of the cubic m3m point group symmetry of fluorite to rhombohedral m point group symmetry in both cases. The type of rhombohedral symmetry, however, was found to be quite distinct. The ʹ1 phase is shown to be a two-dimensional incommensurately modulated phase of R m(00) 11 superspace symmetry while the ʹʹ1 phase is shown to be a rhombohedral, commensurately modulated, superstructure phase. Ca/Bi metal ion ordering on the essentially ccp cation array is shown to be largely responsible for the observed satellite reflections and an expression for this ordering is derived using a modulation wave approach to structural characterization.