Flexoelectric Polarization-Electric Field Coupling-Driven Phase Transformation in Epitaxial Films

Abstract

Nanoscale phase transformations at interfaces enable unprecedented control over the structure and functionalities of low-dimensional materials. Flexoelectric polarization, universally produced by the strain gradient in all dielectrics, can be directly coupled to an external electric field and amplified at nanoscale interfaces. Here, we show that this coupling leads to an electromechanical response and a reversible phase transformation at a nanoscale flexoelectric layer between morphotropic phases. In epitaxial BiFeO₃–BaTiO₃ thin films, the external electric field triggers a transient structural transition from an interfacial flexoelectric layer to crystalline phases, as revealed by in situ time-resolved X-ray microdiffraction. When the electric field exceeds the built-in flexoelectric field, the net interfacial electrical polarization is reconstructed inversely, resulting in a change in the electromechanical response and phase transformation. These results establish flexoelectric coupling at nanoscale interfaces as an alternative, electrically controllable pathway for dynamic phase engineering in complex material systems.