Relationship Between Structure and Ferroelectric Properties

As an important family of electroactive materials, ferroelectrics with the polar structures are characterized by spontaneous polarization, of which the direction can be reversibly switched under external electric field. Ferroelectrics demonstrate diverse physical attributes including piezoelectricity, pyroelectricity, nonlinear optical effect, mechanical functions, and dielectric properties, occupying an indispensable position in the field of condensed matter physics. All these fascinating properties and wide applications of ferroelectric materials are closely related to their unique structural characteristics. Essentially, spontaneous polarization of ferroelectrics can only exist below the Curie temperature point (T c), coupling with the remarkable symmetry breaking during the paraelectric-to-ferroelectric transitions. Various physical properties for ferroelectric materials exhibit giant anomalies in the vicinity of T c, stemming from their structure changes. Compared with traditional inorganic ferroelectrics, the counterpart of molecular ferroelectrics displays many distinct characteristics, such as lightweight, easy processing, mechanical flexibility, tunable structure, biocompatibility, etc. Therefore, molecular ferroelectrics are currently become one of the research hotspots. This section will discuss the relationship between the symmetry-breaking crystal structures and ferroelectric properties, and the recent advance of molecular ferroelectrics is also systematically summarized.