EXAFS and Near Edge Structure IV

J. Phys. Colloques 47 (1986) C8-669-C8-673
DOI: 10.1051/jphyscol:19868126

EXAFS STUDIES OF FLUORITE OXIDES

P.D. BATTLE¹, C.R.A. CATLOW², A.V. CHADWICK³, G. N. GREAVES⁴ et L.M. MORONEY<sup>5

1</sup>  Chemistry Department, Leeds University, GB-Leeds LS2 9JT, Great-Britain
²  Chemistry Department, Keele University, GB-Keele ST5 5AG, Great-Britain
³  Chemistry Department, Kent University, GB-Canterbury CT2 7NH, Great-Britain
⁴  S.E.R.C., Daresbury Laboratory, GB-Daresbury WA4 4AD, Great-Britain
⁵  Brookhaven National Laboratory, Upton, NY 11973, U.S.A.

Abstract
Many of the unique possibilities of EXAFS as a structural technique are illustrated in the problem of determining the local interactions between defects and anion vacancies in the fluorite oxide systems, yttria stabilised zirconia and Y₂O₃-doped Bi₂O₃. From the measured EXAFS on the Y, Zr and Bi absorption edges, it is possible to compare directly the host and dopant cation's different local structural environments. This shows that, in both systems, the Y³⁺ ions adopt a more ordered and isotropic local environment than the host cation. Both Bi³⁺ and Zr⁴⁺ tend to be displaced from their centrosymmetric sites and the static disorder in the nearest neighbour oxygen shell is comparatively extensive. For yttria-stabilised zirconia, this behaviour can be attributed to ionic relaxations in response to anion vacancies in the Zr-O shell, the anion vacancies being necessary for charge compensation of the trivalent Y³⁺ ions. The disorder in the Bi₂O₃ samples is greater than in yttria-stabilised zirconia which can be correlated with the higher anion conductivity in the former. The preference that the host cations exhibit for anisotropic co-ordination geometries can be explained by their high polarisabilities due to the high charge and small size of the Zr⁴⁺ ion and the lone electron pair on the Bi³⁺ ion.