Ordering of Ce-III/Ce-IV and interstitial oxygens in CeTaO4+x (x approximate to 0.17) superstructure

A structural model for CeTaO4+x (x approximate to 0.17) or alternately (C4C e2Ta6O25)-Ce-III-Ta-IV (monoclinic, P2(1), a = 7.616(2), b =16.459(5), c = 7.704(2) Angstrom, beta = 102.48(2)degrees, Z = 2) is derived from X-ray po wder diffraction data. The starting model was a 3 x b superstructure of CeT aO4+x (x = 0.0) with excess oxygen atoms occupying interstices between the Ce atom layers consistent with the electron diffraction determined space gr oup symmetry P2(1). Unexpectedly, the four Ce atoms surrounding the interst itial oxygen are not oxidized. During the refinement the model was forced t o be chemically plausible by restraining bond valence sums, bond lengths, a nd O-O distances. This was necessary due to the very weak scattering contri bution to the XRD profile by oxygen atoms. The mechanism for oxidation of t he subject phase from CeTaO4+x (x = 0.0) is discussed and a reason for the limiting composition for this phase is proposed. (C) 1999 Academic Press.