The compositional and positional disorder of the constituent atoms in crystals with a defect fluorite structure was studied by analyzing the extended X-ray absorption fine structure and by investigating the structure using single-crystal X-ray diffractometry. An extended X-ray absorption fine structure analysis that was based upon the single-shell model yielded mean Y-O and Ta-O bond distances of 0.229 and 0.198nm, respectively. An X-ray diffraction study revealed that the structure was essentially of fluorite type, although the Y atoms were displaced along <111> by 0.0219nm from the Ca fluorite position and half of the O atoms were displaced along <001> by 0.054nm from the F position. The Ta atom was surrounded by at most eight O atoms, with a Ta-O bond distance of 0.201nm, while the Y atom was surrounded by at most seven O atoms, with a mean Y-O bond distance of 0.2277nm. It was concluded that the mean bond distances and the coordination numbers of metal atoms which were found by means of X-ray diffraction were generally consistent with those which were found by using the extended X-ray absorption fine structure technique. Most of the O vacancies in this material appeared to concentrate around Y atoms. The local structure around Y atoms here was similar to that in Y2O3. The crystal data for the present material indicated that a = 0.52553nm (Fm3m) at 24C.

T.Tanaka, N.Ishizawa, M.Yoshimura, F.Marumo, H.Oynagi: Journal of Solid State Chemistry, 1995, 114[1], 79-87