Hybrid Improper Ferroelectricity and Tunable Ionic States in Bilayer Perovskites

Ferroelectricity was first discovered in 1920. The intrinsic binary states and electrically switchable nature enable wide applications of ferroelectric materials in electronics. Until now, the investigation of new ferroelectrics, unconventional ferroelectric mechanisms, and the coupling between ferroelectricity with other physical properties are still important topics in condensed matter physics. In bilayer perovskites, ferroelectricity can be induced by a combination of octahedron rotation and tilting, i.e., the hybrid improper ferroelectricity mechanism. In the first part of this talk, I will present the experimental demonstration of hybrid improper ferroelectricity in bilayer perovskite Sr₃Sn₂O₇ single crystal. The switching barrier is found to be remarkably low, resulting in a ferroelastic-coupled domain wall mobility. Theoretical and experimental evidence of a metastable antipolar intermediate phase as the possible root cause of low switching barrier will be discussed. The second part will focus on experimental studies of bilayer perovskite Li₂SrNb₂O₇ single crystal. Electron diffraction reveals a complex structure evolution, leading to anisotropic and temperature-strongly-dependent dielectric response. Moreover, due to intralayer Li ion activities, this material acts as a (anti)ferroelectric insulator or an ionic conductor (with memristive behavior and rectification effect) in different temperature and frequency regimes. These findings provide insight into the application of tunable ionic states in multifunctional quantum materials.