title: Structure-property relationship of supramolecular ferroelectric [H-66dmbp][Hca] accompanied by high polarization, competing structural phases, and polymorphs.
abstract: 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, P21) into an anti-polar structure (space group, P21/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 (TC ) 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 (Ps ) 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.
authors.affiliation: Condensed Matter Research Center (CMRC) and Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba 305-0801 (Japan), Fax: (+81) 29-864-3202. kensuke.kobayashi@kek.jp.
authors.forename: Kensuke
authors.initials: K
authors.lastname: Kobayashi
authors.forename: Sachio
authors.initials: S
authors.lastname: Horiuchi
authors.forename: Shoji
authors.initials: S
authors.lastname: Ishibashi
authors.forename: Fumitaka
authors.initials: F
authors.lastname: Kagawa
authors.forename: Youichi
authors.initials: Y
authors.lastname: Murakami
authors.forename: Reiji
authors.initials: R
authors.lastname: Kumai
keywords: hydrogen bonds; organic ferroelectrics; phase transition; supramolecular chemistry