Plastic/ferroelectric molecular crystals: Ferroelectric performance in bulk polycrystalline forms

Plastic crystals are a class of compounds composed of small molecules with globular structures. Plastic crystals exhibit various unique features, most of which arise from the rotator motion and orientational disorder of the constituent molecules. We recently discovered that some plastic crystals exhibit ferroelectricity in lower-temperature phases, including the room-temperature phase. The crystals, i.e., plastic/ferroelectric molecular crystals, have demonstrated several unique features that differ from conventional molecular ferroelectrics. In particular, the materials have achieved ferroelectric performance for the first time as bulk polycrystals of small-molecule crystals. These features are attributed to the multiaxial ferroelectricity and facile preparation of monolithic bulk polycrystals, which are impossible to achieve for conventional molecular crystals. Numerous related molecular ferroelectrics have been developed, rendering plastic/ferroelectric crystals an emerging class of molecular ferroelectrics. In this perspective, we have outlined the development and unique features of plastic/ferroelectric molecular crystals, focusing particularly on their ferroelectricity and related properties in bulk polycrystalline forms.