Molecular Crystal Project – Precise crystal structure analysis for investigating the origin of physical properties in molecular crystals – Project Leader : Reiji Kumai

Ferroelectricity is a key issue in condensed matter physics. In ferroelectric materials, the application of an external electric field, E, can switch the spontaneous polarization through the inverting polarity of the noncentrosymmetric crystal structure. By making use of such property, ferroelectric materials can be applied in various electric applications such as memory, sensor, actuator, and optical devices. Most of these devices are made from inorganic ferroelectrics such as Pb(Ti,Zr)O3 (PZT), SrBi2Ta2O9, BaTiO3, and LiNbO3 because their operation in ambient environments demands a large spontaneous polarization (Ps) and high Curie temperature (TC) of a paraelectricto-ferroelectric phase transition near or above room temperature. Recently, organic ferroelectric materials containing neither toxic nor rare elements have attracted significant attention due to their flexible, environmentally friendly, fusible, and low-cost characteristics. Specifically, singlecomponent hydrogen-bonded compounds such as croconic acid and benzimidazoles have led to a significant improvement in the Ps and TC up to far above room temperature. The structural and electronic mechanisms of ferroelectricity in this organic system can involve intramolecular and intermolecular processes, and can lead to characteristics distinct from those of inorganic ionic ferroelectrics. Structural characterizations are indispensable for understanding the microscopic mechanism of ferroelectricity, which may be useful for advancing the design of various materials. For instance, X-ray diffraction studies on poled crystals of tetrathiafulvalene (TTF)-p-chloranil (CA) complex revealed that unexpectedly large electric polarization originates from intermolecular charge-transfer processes and consequently is directed antiparallel to the static charge (ionic) displacement. In this study, we examined the structure– property relationship of ferroelectric binarycompound crystals, in which hydrogen-bonding forms a one-dimensional supramolecular chain of alternating acid and base molecules. Using a series of anilic acids, the polarity of the protontransferred monovalent ionic form can be switched through collective proton transfer between neighboring acid and base molecules. Many of these supramolecular crystals undergo structural phase transitions related to proton order-disorder phenomena. As compared to the aforementioned single component materials, ferroelectric supramolecules were reported to exhibit modest TC and Ps until recent findings regarding 6,6′-dimethyl-2,2′bipyridinium chloranilate [H-66dmbp][Hca] (Fig. 1). This compound, which has the longest hydrogen bond length among organic ferroelectric supramolecules, was first reported by Bator et al.[2] A spontaneous polarization Ps of 7–8 μC cm was about half of that of croconic acid (~21 μC cm) and comparable to that (~10 μC cm) of 3 Molecular Crystal Project