Ordering of CeIII/CeIVand Interstitial Oxygens in CeTaO4+x(x≈0.17) Superstructure

A structural model for CeTaO4+x(x≈0.17) or alternately CeIII4CeIV2Ta6O25(monoclinic,P21,a=7.616(2),b=16.459(5),c=7.704(2) Å,β=102.48(2)°,Z=2) is derived from X-ray powder diffraction data. The starting model was a 3×bsuperstructure of CeTaO4+x(x=0.0) with excess oxygen atoms occupying interstices between the Ce atom layers consistent with the electron diffraction determined space group symmetryP21. Unexpectedly, the four Ce atoms surrounding the interstitial oxygen are not oxidized. During the refinement the model was forced to be chemically plausible by restraining bond valence sums, bond lengths, and O–O distances. This was necessary due to the very weak scattering contribution to the XRD profile by oxygen atoms. The mechanism for oxidation of the subject phase from CeTaO4+x(x=0.0) is discussed and a reason for the limiting composition for this phase is proposed.