Satellite dark-field imaging was used to show that there was a definite change in symmetry upon moving across the 2-phase region which separated the so-called defect-fluorite and C-type sesqui-oxide solid-solution regions in these systems. Satellite dark-field images of the defect-fluorite side of the 2-phase region were typified by a micro-domain texture at the scale of 10 to 20nm, and the local symmetry within each micro-domain was lower than cubic. The corresponding satellite dark-field images of the C-type sesqui-oxide side of the 2-phase region were relatively homogeneous and were consistent with an Ia3 space-group symmetry. The absence of ¼{220}*-type satellite reflections in <001> zone-axis electron diffraction patterns on either side of the 2-phase region, implied that ¼{220}* modulation waves could be associated only with O vacancy ordering and an associated structural relaxation. The latter was considered to be a response to the O vacancy distribution. The absence of contrast in satellite dark field images on the C-type side of the 2-phase region showed that the nature of the local O vacancy distribution on this side of the 2-phase region was characteristic of the end-member C-type sesqui-oxide structure itself. This Ia3 structure had O vacancies which were arranged so that there were no ½<100> O vacancy pairs around filled metal-atom sites. However, the large proportion (25%) of O vacancies permitted the existence of ½<110> vacancy pairs around filled metal atom sites and of ½<111> O vacancy pairs across empty cubes.

R.L.Withers, J.G.Thompson, N.Gabbitas, L.R.Wallenberg, T.R.Welberry: Journal of Solid State Chemistry, 1995, 120[2], 290-8