Raman Scattering in La$_{0.2}$Sr$_{0.8}$FeO$_{3-\delta}$ thin film: annealing-induced reduction and phase transformation

Raman scattering in thin film La$_{0.2}$Sr$_{0.8}$FeO$_{3-\delta }$ on MgO(001) collected at 300 K following different stages of annealing at selected temperatures (300 K \textless T \textless 543 K, to 10 h.) and analysis reveal changes in spectral characteristics due to loss of oxygen, onset of oxygen vacancy-induced disorder, and activation of Raman-inactive modes that are attributed to symmetry transformation. The interpretation is further supported by carrier transport measurements under identical conditions showing orders of magnitude increase in the resistivity induced by oxygen loss. After prolonged annealing in air, evolution of the spectrum is consistent with the appearance of a topotactic transformation of the crystal structure from that of the rhombohedral \textit{AB}O$_{3}$ perovskites to that of Brownmillerite-like structure consisting of octahedrally and tetrahedrally coordinated Fe atoms.