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. 2017 Aug 17;8:281. doi: 10.1038/s41467-017-00250-y

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© The Author(s) 2017

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Fig. 1 — Molecular structures and neutral-to-ionic phase transition in TTF-CA. a Molecular structure of tetrathiafulvalne (TTF) and p-chloranil (CA) and the mixed stack structure of the two molecules along the a-axis. TTF is an electron donor (D) and CA is an electron acceptor (A). b Schematic electronic structures of TTF-CA in neutral and ionic phases. In the ionic phase, the dimer formation induces spontaneous polarization. The polarization has ionic and electronic contributions (P = P _ion + P _el). The former originates from the displacement of charged molecules, while the latter originates from the charge transfer between D–A molecules. In TTF-CA, P _el is much larger than P _ion and their directions are opposite. c Temperature dependence of the spontaneous polarization determined from the pyroelectric current measured after the sample was cooled under poling fields (E _pole) of ±2 kV cm^–1 and without the poling procedure (E _pole = 0)