The nature of polar distortions in ferroelectrics

Abstract: Polar distortion, the collective off-center displacements of atoms, is a fingerprint of a ferroelectric that governs its properties and functionalities. Since the 1970s, the concepts of proper, improper and triggered ferroelectrics have been established to shed light on a diversity of polar distortion mechanisms. Such concepts assign a single nature to polar distortion and are helpful to interpret how polar distortions occur in conventional ferroelectrics such as barium titanate. However, applying these concepts to complex ferroelectrics (e.g., polar orthorhombic hafnia) is notoriously challenging and can yield highly controversial arguments. Here we resolve this issue by developing a tailor-made graph theory for clarifying the nature of polar distortions in complex ferroelectrics, which emphasizes that polar distortions in such ferroelectrics usually exhibit multiple natures among proper, improper and triggered characteristics. We demonstrate the robustness of our theory by working with perovsktie superlattices and polar orthorhombic hafnia (i.e., two representative cases). We successfully identify the mixed proper-improper nature in perovsktite superlattices and reconcile the controversy on polar orthorhombic hafnia by confirming its mixed trigger-improper nature. Our work will definitely lead to a revisitation of concepts in ferroelectric physics and provide opportunities for discovering novel ferroelectrics and related phenomena.