Freestanding Perovskite and Brownmillerite SrCoO_<i>x</i> Membranes for Direct Imaging of Topotactic Phase Transitions

Topotactic phase transitions between perovskite SrCoO₃ and brownmillerite SrCoO_2.5 have garnered attention for application in energy-efficient neuromorphic computing systems. However, the evolution of local microstructures, crucial for designing topotactic devices and improving switching performance, remains underexplored due to the limitations of epitaxial thin films bound to bulk substrates. Here, we report the synthesis and characterization of topotactic oxide SrCoO_<i>x</i> as a freestanding membrane, successfully stabilizing both perovskite and brownmillerite phases. Plan-view transmission electron microscopy identifies the extraordinary formation of filamentary one-dimensional (1D) domains of SrCoO_2.5 phases during the topotactic phase transition, correlated with the 1D oxygen vacancy channels that facilitate rapid oxygen ion diffusion. These 1D domains serve as direct evidence of the anisotropic nature of the topotactic phase transition, dictated by oxygen diffusion. These findings also underscore the advantages of freestanding membrane geometries for developing a fundamental understanding of phase transitions in complex oxides.