A SINGLE-CRYSTAL X-RAY STUDY OF THE FLUORITE-RELATED SOLID-SOLUTIONS OF CEO2-YO1.5

A study of the (1 - delta)CeO2 .delta YO1.5 solid solution has been un dertaken to understand the origin of the two-phase region that occurs between the fluorite-type, 0- similar to 50 mole% YO1.5, and C-type, s imilar to 73-100 mole% YO1.5, solid solutions. Single crystal X-ray di ffraction data have been collected at 49.7, 73.0, and 79.8 mole% YO1.5 (cubic, Ia ($) over bar 3, a = 10.757(2), 10.698(4), 10.678(3) Angstro m, Z = 32) and the structures refined to R(1) = 0.0841, 0.0146, and 0. 0202 for 29, 25, and 72 independent reflections, respectively. The dev iations from the ideal fluorite structure were examined and an apparen t discontinuity observed for the metal atom shift allowed by Ia ($) ov er bar 3 symmetry. Previously suggested metal atom ordering was not ob served in the C-type structure. Models have been proposed to understan d the limiting compositions of the fluorite-type and C-type solid solu tions. (C) 1995 Academic Press, Inc.