Abstract
Periodic DFT calculations allow an understanding of the strong orientation-dependent Ramanspectra of oriented CaFeO2.5 single crystals. Modes involving the oscillation of the apical oxygen(Oap) atoms perturb the induced electric dipoles. These are formed by anisotropy in the chargedistribution and are found to be strongly enhanced when the electric field of the linearly polarizedlaser line is parallel to the b axis. For the CaFeO2.5 ordered system, strong polarizability of thesemodes corresponds to strong Raman intensities. Conversely, the apical oxygen disorder observedin low-temperature oxygen-conducting SrFeO2.5 destroys the long-range coherence of therespective Raman modes, which consequently show a strongly reduced intensity. This studyprovides a vibrational tool to discriminate between ordered and disordered isomorporphousABO2.5 Brownmillerite frameworks. Furthermore, in combination with DFT calculations, we havefound that the weakening of the interlayer interactions is responsible for the loss of ordering inBrownmillerite compounds