In situ Negative Cs HRTEM Imaging of Topotactic Phase Transformation from Perovskite SrFeO₃ to Brownmillerite SrFeO_2.5

SrFeO3, a candidate electrode material for solid oxide fuel cell, undergoes topotactic phase transition reversibly to SrFeO2.5 brownmillerite at relatively low temperatures. It has been known that during the topotactic phase transition the cation sub-lattice remains almost unchanged while oxygen ions (or vacancies) diffuses through the lattice along the specific crystallographic channels to modify the local coordination and valence states of cations. Previous x-ray diffraction [1] and sequential STEM HAADF imaging have been used to characterize the phase transition of this type of materials. However, neither of these methods can directly observe the channeling of oxygen vacancies, which is challenging due to the low scattering power of oxygen ions.