Solid Solutions of Plastic/Ferroelectric Crystals: Toward Tailor-Made Functional Materials

Plastic crystals that show ferroelectricity are highly promising materials for a wide range of applications. Their inherent remarkable malleability and highly symmetric cubic structures in the plastic crystal phase ensure that their ferroelectricity and related properties are retained in their bulk polycrystals. To develop functional materials based on such plastic/ferroelectric crystals, methods to tune their properties for specific applications are required. Here, we report the preparation of solid solutions of plastic/ferroelectric ionic crystals by mixing crystals with a common anion but different cations, or crystals with a common cation but different anions, which allows a continuous modification of the Curie temperature of the ferroelectric system over a range of 100 K. This adjustment of the Curie temperature allows the flexible tuning of the pyroelectric properties of the solid solutions, including a significant enhancement of room-temperature performance. The solid solutions also exhibit morphotropic phase boundaries in the composition-temperature phase diagrams, which shows an abrupt change in crystal structures with a variation of composition. This study showcases a simple and versatile property-tuning method that can be expected to pave the way for major progress in the development of materials based on plastic/ferroelectric crystals, which will eventually advance to the stage of pursuing tailor-made functional materials with desired properties.