Microdomains, Solid Solutions and the "Defect Fluorite" to C-Type Sesquioxide Transition in CeO2-RO1.5 and ZrO2-RO1.5 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 100-200 Å 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.