Microdomains, solid solutions and the ''defect fluorite'' to C-type sesquioxide transition in CeO2-RO(1.5) and ZrO2-RO(1.5) systems

Satellite dark field (SDF) imaging is used to show that there is a definite change in symmetry on moving across the two-phase region separating the so-called ''defect fluorite'' and C-type sesquioxide solid solution regions in (1 - x)CeO2 . xRO(1.5) and (1 - x)ZrO2 . xRO(1.5) systems, SDF images of the ''defect fluorite'' side of the two-phase region are characterized by a microdomain texture on the similar to 100-200 Angstrom scale and the local symmetry within any one of these microdomains is shown to be lower than cubic. Corresponding SDF images of the C-type sesquioxide side of the two-phase region are by contrast homogeneous and consistent with Ia3 space group symmetry. The nature of the local oxygen vacancy distribution on either side of the two-phase region is discussed and a possible model for the ''defect fluorite'' side of the two-phase region proposed. (C) 1995 Academic Press, Inc.