AN ELECTRON-DIFFRACTION STUDY OF SHORT-RANGE ORDER IN THE (1-X)BI2O3-CENTER-DOT-XCAO GAMMA-TYPE SOLID-SOLUTION AND ITS RELATIONSHIP TO LOW-TEMPERATURE CA4BI6O13

The body centered cubic (a similar to 4.26 Angstrom) gamma-type solid solution (ss gamma) phase (1-x)Bi2O3 . xCaO (similar to 0.50 < x < sim ilar to 0.60 at similar to 820 degrees C) was investigated by electron diffraction, Instead of finding evidence for additional Bragg reflect ions at G+/-a as previously assumed attempting to interpret powder ne utron diffraction data (Gruber et al., J, Solid State Chem. 108, 250-2 52, 1994), localized diffuse scattering was detected and described by G+/-q, where q approximate to 0.2[211](gamma) is a (diffuse) modulati on wave vector. A crystal structural relationship between the cation a rrays in the ss gamma phase and the low-temperature line phase, Ca4Bi6 O13 (LT), was established, The unit cell of the LT phase can be expres sed as a(LT)=a(gamma)-c(gamma), b(LT)=5/3(a(gamma)-2b(gamma)+c(gamma)) and c(LT)=1/2(a(gamma)+b(gamma)+c(gamma)). From the cation ordering i n the LT phase and the reciprocal space relation 2b(LT)=0.2[1 (2) ove r bar 1](gamma)approximate to q, the localized diffuse scattering in the ss gamma phase was linked to cation ordering into layers perpendic ular to [1 (2) over bar 1](gamma). (C) 1998 Academic Press.