title: Electronic ferroelectricity in a molecular crystal with large polarization directing antiparallel to ionic displacement.
abstract: Ferroelectric polarization of 6.3  μC cm(-2) is induced by the neutral-to-ionic transition, upon which nonpolar molecules of electron donor tetrathiafulvalene (TTF) and acceptor p-chloranil (CA) are incompletely ionized to ±0.60e and dimerized along the molecular stacking chain. We find that the ferroelectric properties are governed by intermolecular charge transfer rather than simple displacement of static point charge on molecules. The observed polarization and poling effect on the absolute structural configuration can be interpreted in terms of electronic ferroelectricity, which not only exhibits antiparallel polarity to the ionic displacement but also enhances the polarization more than 20 times that of the point-charge model.
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. kensuke.kobayashi@kek.jp
authors.forename: Kensuke
authors.initials: K
authors.lastname: Kobayashi
authors.forename: Sachio
authors.initials: S
authors.lastname: Horiuchi
authors.forename: Reiji
authors.initials: R
authors.lastname: Kumai
authors.forename: Fumitaka
authors.initials: F
authors.lastname: Kagawa
authors.forename: Youichi
authors.initials: Y
authors.lastname: Murakami
authors.forename: Yoshinori
authors.initials: Y
authors.lastname: Tokura