Anisotropy in the Raman scattering of a CaFeO2.5 single crystal and its link with oxygen ordering in Brownmillerite frameworks

Periodic DFT calculations allow understanding the strong orientation dependent Raman spectra of oriented CaFeO2.5 single crystals. Modes involving the oscillation of the apical oxygen (Oap) atoms were found to be strongly enhanced when the electric field of the linearly polarized laser line is parallel to b. For CaFeO2.5 ordered system strong polarizability of these modes go along with strong Raman intensities. Conversely, the apical oxygen disorder observed in oxygen conduction SrFeO2.5 destroys the long range coherence of the respective Raman modes, which consequently show a strongly reduced intensity. This study provides a vibrational tool to discriminate between ordered and disordered isomorporphous ABO2.5 Brownmillerite frameworks. Furthermore, in combination with DFT calculations, we found that the weakening of the interlayer interactions is responsible for the loss of ordering in some of the Brownillerite compounds.