EXAFS STUDIES OF FLUORITE OXIDES

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.