An electron diffraction study of short-range order in the (1-x)Bi2O3•xCaO γ-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 < similar to 0.60 at similar to 820 degrees C) was investigated by electron diffraction, Instead of finding evidence for additional Bragg reflections at G+/-a* as previously assumed attempting to interpret powder neutron diffraction data (Gruber et al., J, Solid State Chem. 108, 250-252, 1994), localized diffuse scattering was detected and described by G+/-q, where q approximate to 0.2[211](gamma)* is a (diffuse) modulation wave vector. A crystal structural relationship between the cation arrays in the ss gamma phase and the low-temperature line phase, Ca4Bi6O13 (LT), was established, The unit cell of the LT phase can be expressed 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 in the LT phase and the reciprocal space relation 2b(LT)*=0.2[1 (2) over bar 1](gamma)*approximate to q, the localized diffuse scattering in the ss gamma phase was linked to cation ordering into layers perpendicular to [1 (2) over bar 1](gamma). (C) 1998 Academic Press.