A structural model for CeTaO4.17 or Ce4IIICe2IVTa6O25 (monoclinic P21, a = 0.7616, b = 1.6459, c = 0.7704nm, β = 102.48º, Z = 2) was deduced from X-ray powder diffraction data. The starting model was a 3 x b superstructure of CeTaO4, with excess O atoms occupying the interstices between the Ce-atom layers; so as to be consistent with the electron diffraction-indicated space-group symmetry of P21. However, the four Ce atoms which surrounded the interstitial O atom were not oxidized. During refinement, the model was made more chemically plausible by restraining the bond-valence sums, bond lengths and O-O distances. This was necessary because of the very weak scattering contribution which was made to the X-ray diffraction profile by O atoms.

Ordering and Interstitial Oxygen in CeTaO4 Superstructure. J.G.Thompson, A.D.Rae, N.Bliznyuk, R.L.Withers: Journal of Solid State Chemistry, 1999, 144[2], 240-6