THE DEFECT STRUCTURE OF SR2FETIO6-X

The nature of the defect structure of the nonstoichiometric perovskite Sr//2FeTiO//6// minus //x has been investigated. Powder X-ray diffraction and electron diffraction measurements imply a simple cubic perovskite unit cell with no long-range vacancy order, and Mossbauer spectra are interpreted using short-range order models. These are related to the structure of Sr//2Fe//2O//5 and contain strings of oxygen vacancies which generate four-coordinate Fe**3** plus along their lengths and terminate at five-coordinate Fe**3** plus . Oxygen and iron displacements, revealed by powder neutron diffraction, are shown to be consistent with local distortions to provice an approximately tetrahedral environment for the four-coordinate species. The models are also discussed in relation to SrFeO//3// minus //x.