An Electron Diffraction Study of Short-Range Order in the (1-x)Bi2O3·xCaOγ-Type Solid Solution and Its Relationship to Low-Temperature Ca4Bi6O13

Abstract

The body centered cubic (a~4.26 Å)γ-type solid solution (ssγ) phase (1-x)Bi2O3·xCaO (~0.50<x<~0.60 at ~820°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 (Gruberet al., J. Solid State Chem.108, 250-252, 1994), localized diffuse scattering was detected and described by G±q, where q≈0.2〈211〉*γis a (diffuse) modulation wave vector. A crystal structural relationship between the cation arrays in the ssγphase and the low-temperature line phase, Ca4Bi6O13(LT), was established. The unit cell of the LT phase can be expressed as aLT=aγ-cγ, bLT=53(aγ-2bγ+cγ) and cLT=12(aγ+bγ+cγ). From the cation ordering in the LT phase and the reciprocal space relation 2b*LT=0.2〈121〉* γ≈q, the localized diffuse scattering in the ssγphase was linked to cation ordering into layers perpendicular to 〈121〉γ. Show more