Horiuchi Sachio, Ishibashi Shoji, Kobayashi Kensuke, Kumai Reiji
Electronics and Photonics Research Institute (ESPRIT), National Institute of Advanced Industrial Science and Technology (AIST) Tsukuba Ibaraki 305-8565 Japan.
Research Center for Computational Design of Advanced Functional Materials (CD-FMat), National Institute of Advanced Industrial Science and Technology (AIST) Tsukuba Ibaraki 305-8568 Japan.
RSC Adv. 2019 Dec 2;9(68):39662-39673. doi: 10.1039/c9ra06489c.
Supramolecular cocrystals of anilic acids with 2,2'-bipyridines exhibit successive phase transitions as well as unusual isotope effects. Ferroelectricity driven by a cooperative proton transfer along the supramolecular chains is accompanied by huge permittivity (a maximum of 13 000) at the Curie point, as well as a large spontaneous polarization (maximum 5 μC cm) and a low coercive field ranging from 0.5 to 10 kV cm. Deuterium substitutions over the hydrogen bonds smoothly raise the Curie point and simultaneously reduce other phase-transition temperatures by a few tens of degrees. The coexistence of opposite isotope effects reduces the temperature interval of the intermediate paraelectric phase from 84 to 10 K for the 5,5'-dimethyl-2,2'-bipyridinium bromanilate salt. The bipyridine molecules exhibit interplanar twisting, which represents the order parameter relevant to the high-temperature phase transitions. The normal and inverse temperature shifts are ascribed to the direct and indirect effects, respectively, of the lengthened hydrogen bonds, which adjusts the molecular conformation of the flexible bipyridine unit so as to minimally modify their adjacent intermolecular interactions.
苯胺酸与2,2'-联吡啶形成的超分子共晶体表现出连续的相变以及异常的同位素效应。沿着超分子链的协同质子转移驱动的铁电现象伴随着居里点处的巨大介电常数（最大值为13000），以及大的自发极化（最大值为5 μC/cm²）和0.5至10 kV/cm的低矫顽场。氢键上的氘取代使居里点平稳升高，同时使其他相变温度降低几十度。对于5,5'-二甲基-2,2'-联吡啶溴苯胺盐，相反同位素效应的共存将中间顺电相的温度区间从84 K减小到10 K。联吡啶分子表现出平面间扭曲，这代表了与高温相变相关的序参量。正温度偏移和负温度偏移分别归因于氢键延长的直接和间接效应，氢键延长会调整柔性联吡啶单元的分子构象，从而最小程度地改变其相邻的分子间相互作用。
