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

Abstract

The transition from isolated and finite oxygen vacancy clusters in disordered cubic perovskites to structures containing infinite chains of oxygen vacancies in an ordered arrangement was investigated by a combined Mössbauer spectroscopy, X-ray and electron diffraction and high-resolution transmission electron microscopic study. With increasing defect concentration (equivalent to an increase in the CaFeO_2.5 component concentration) and/or decreasing 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 orientational disorder with two different directions of vacancy chains in addition to stacking disorder with variable sequences of tetrahedral and octahedral layers. At higher vacancy concentrations, ordered layer sequences with proportions defined by stoichiometry are formed. The ordering of vacancies into infinite chains and sheets leads to a large and discontinuous decrease in ionic conductivity.