Hydrogen-Bonded Small-Molecular Crystals Yielding Strong Ferroelectric And Antiferroelectric Polarizations

The development of organic ferroelectrics and antiferroelectrics with significantly improved performances has recently increased. Their attractive features are environmentally benign nature and technological advantages such as the light weight, flexibility, biocompatibility, and low-temperature processability. This review is dedicated to the 100th anniversary of the discovery of the first ferroelectric material, Rochelle salt, which contains hydrogen-bonded organic molecules. Approximately one third of more than 80 ferroelectric and 30 antiferroelectric small-molecular crystals are hydrogen-bonded (prototropic, acid-base supramolecular, and zwitterionic) proton-transfer-type crystals. The synergistic interplay of the intermolecular proton dynamics and switchable p-bond dipoles strengthens the switchable polarization and thermal stability, as represented by croconic acid with the highest spontaneous polarization among organic ferroelectrics. Highly polarizable antiferroelectric counterparts are also in the focus, especially the high density and highly efficient electrostatic energy storage of squaric acid and the giant electrostriction of naphthimidazole. A structural assessment using a database is the methodology hidden behind many of the latest successful developments. Theoretical investigations on organic (anti)ferroelectrics using the Berry phase formalism provide the design principles and strategies that can be used to obtain strong polarizations such as achieved for prototropic ferroelectrics. In addition to hydrogen bonds, the latest discovery of molecular rotational ferroelectric crystals (including plastic/ferroelectric crystals) represents a breakthrough with respect to material design strategies that can be used to overcome the bottlenecks inherent to the organic system to simultaneously achieve a high performance, endurance, and adaptability to target devices. Multiaxially switchable strong polarizations arise from both the order-disorder of dipoles and ionic displacement mechanisms, as represented by a metal-free perovskite-type ferroelectric. Therefore, recent advances indicate the availability of further key strategies as well as the synergetic effects of multiple degrees of freedom in molecular crystals.