Local polar−antipolar ground-state interconversion in La-substituted BiFeO₃

Over the last decade, advances in the design of electrically ordered materials have led to exciting observations of electrically tunable topological polar textures, antipolar ordering, and incommensurate polar states. The ability to reversibly convert between such phases and hence select among different polarization states would provide a rich playground for the development of multifunctional devices. Here, we demonstrate the local interconversion between the as-grown ferroelectric and a novel antipolar phase in La-substituted BiFeO₃ films. We trigger the remanent phase transition from the ferroelectric to the antipolar Pnma phase through the application of stress with a scanning-probe tip and show the full recovery of the ferroelectric order solely by the application of an electric field. Additional atomic-scale investigations reveal that the antipolar phase is stabilized by an A-site ordering of the La ions in the BiFeO₃ perovskite structure. By tuning the applied stress, we can control the magnitude of the polarization almost continuously in between the two saturation polarization states. Moreover, our non-invasive studies using optical second-harmonic generation reveal that we can induce additional polarization states beyond the conventional binary response of a ferroelectric material in capacitor heterostructures using stress and electric fields. Our results therefore promote the development of entirely new concepts for multifunctional devices based on the additional functionality dimension presented by the deterministic control over the polar-antipolar phase coexistence.