Phonon decoupling in oxides

Brownmillerite has emerged as a promising candidate for innovative ferroelectric applications, and it can be obtained by topotactically reducing the perovskite structure. Due to the distinct alignment of oxygen vacancy channels, unusual properties manifest, including ferroelectricity. Theoretically, BO₄ tetrahedral distortion could induce inversion symmetry breaking within the crystal structure, and both ionic displacement and rotation of the FeO₄ tetrahedra can be combined in brownmillerite SrFeO_2.5 (SFO) and CaFeO_2.5 (CFO), revealing a unique one-dimensional collective distortion. In these intriguing brownmillerite structures, phonons exhibit unique behaviors that have not yet been fully explored. This study investigates the decoupling of phonon modes associated with oxygen-octahedra from those with oxygen-tetrahedra in brownmillerite oxides. We find that such localized oxygen-tetrahedral phonons enable site-selective control of the unit cell-wide domain. By combining simple 2D modelling and density functional theory (DFT) calculations, we have uncovered the underlying mechanism responsible for phonon decoupling. Our findings demonstrate that specific structural characteristics within the brownmillerite framework lead to the independent vibration of oxygen-octahedra and oxygen-tetrahedra. In this presentation, we will detail our results regarding 2D modeling, phonon band structure, and the implications for ferroelectricity in brownmillerite oxides.