The transition from short-range to long-range ordering of oxygen vacanciesin CaFexTi1-xO3-x/2 perovskites

The transition from isolated and finite oxygen vacancy clusters in disorder ed cubic perovskites to structures containing infinite chains of oxygen vac ancies in an ordered arrangement was investigated by a combined Mossbauer s pectroscopy, X-ray and electron diffraction and high-resolution transmissio n electron microscopic study. With increasing defect concentration (equival ent to an increase in the CaFeO2.5 component concentration) and/or decreasi ng annealing temperature, a disorder-order phase transition occurs leading to infinite chains of vacancies in the [101](cubic) direction, arranged in sheets containing essentially tetrahedral iron. In the early stages of the ordering process and at low iron concentrations, these sheets exhibit orien tational disorder with two different directions of vacancy chains in additi on to stacking disorder with variable sequences of tetrahedral and octahedr al layers. At higher vacancy concentrations, ordered layer sequences with p roportions defined by stoichiometry are formed. The ordering of vacancies i nto infinite chains and sheets leads to a large and discontinuous decrease in ionic conductivity.