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Structure–Property Relationship of Supramolecular Ferroelectric [H‐66dmbp][Hca] Accompanied by High Polarization, Competing Structural Phases, and Polymorphs
Author(s) -
Kobayashi Kensuke,
Horiuchi Sachio,
Ishibashi Shoji,
Kagawa Fumitaka,
Murakami Youichi,
Kumai Reiji
Publication year - 2014
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201404759
Subject(s) - ferroelectricity , hydrogen bond , supramolecular chemistry , crystallography , thermal stability , materials science , polarization (electrochemistry) , crystal structure , differential scanning calorimetry , phase transition , chemistry , chemical physics , dielectric , molecule , condensed matter physics , organic chemistry , thermodynamics , physics , optoelectronics
Three polymorphic forms of 6,6′‐dimethyl‐2,2′‐bipyridinium chloranilate crystals were characterized to understand the origin of polarization properties and the thermal stability of ferroelectricity. According to the temperature‐dependent permittivity, differential scanning calorimetry, and X‐ray diffraction, structural phase transitions were found in all polymorphs. Notably, the ferroelectric α‐form crystal, which has the longest hydrogen bond (2.95 Å) among the organic acid/base‐type supramolecular ferroelectrics, transformed from a polar structure (space group, P 2 1 ) into an anti‐polar structure (space group, P 2 1 / c ) at 378 K. The non‐ferroelectric β‐ and γ‐form crystals also exhibited structural rearrangements around hydrogen bonds. The hydrogen‐bonded geometry and ferroelectric properties were compared with other supramolecular ferroelectrics. A positive relationship between the phase‐transition temperature ( T C ) and hydrogen‐bond length (< d >) was observed, and was attributed to the potential barrier height for proton off‐centering or order/disorder phenomena. The optimized spontaneous polarization ( P s ) agreed well with the results of the first‐principles calculations, and could be amplified by separating the two equilibrium positions of protons with increasing < d >. These data consistently demonstrated that stretching < d > is a promising way to enhance the polarization performance and thermal stability of hydrogen‐bonded organic ferroelectrics.