• Brownmillerite;
  • extended defects;
  • oxygen diffusion;
  • perovskites;
  • transmission electron microscopy

We investigated the formation of extended defects in CaFeO2.5, predominantly appearing as antiphase boundaries (APBs), as a function of the synthesis method and temperature. While CaFeO2.5 is known to adopt an ordered oxygen defect structure showing long range order of the (FeO4) chains in its bulk form, interestingly, we demonstrated that the length of these (FeO4) chains can be considerably scaled down to few nanometers by adopting a modified sol–gel method (low temperature synthesis) while the grain size of the resulting nano-phase CaFeO2.5 is around 50 nm. We discuss the synthesis dependent modulation of the length of APBs, characterized by X-ray diffraction and high resolution TEM, to be at the origin of an amplified switching dynamics of the (FeO4) chains. This can accordingly explain the reduction of the onset temperature for oxygen diffusion to set in from 450 °C for bulk-CaFeO2.5 to 320 °C for nano-CaFeO2.5, as determined by 18O/16O oxygen isotope exchange reactions.