Plastic/Ferroelectric Crystals with Distorted Molecular Arrangement: Ferroelectricity in Bulk Polycrystalline Films through Lattice Reorientation

Plastic/ferroelectric crystals have attracted increasing attention as emerging functional molecular materials with unique features. In this study, a new plastic/ferroelectric crystal is developed from ionic molecules, which exhibits three solid phases: a paraelectric plastic crystal phase and two ferroelectric phases. Owing to the malleability in the plastic crystal phase, by applying pressure to the powder at a high temperature, transparent polycrystalline films are easily fabricated, exhibiting ferroelectric performance at room temperature. The crystal structures of the three solid phases are determined by single-crystal X-ray diffraction analysis. The arrangement of anions and cations in the ferroelectric crystal is significantly distorted from the CsCl-type structure in the paraelectric cubic crystal. The large distortion results in distinct splitting of the diffraction peaks that are equivalent in the cubic phase, which enables the observation of electric-field-induced lattice reorientation using powder X-ray diffraction measurements. These results broaden the range of compounds for which plastic/ferroelectric crystals can be developed and provide a solid understanding of the switching process in their bulk polycrystals, which will facilitate their application to a variety of device elements.