Syntheses and Properties of Organic Ferroelectrics Based on Strong Electron-Lattice Interaction

在这项研究中，我们提出了具有低维铁电性的有机pI分子固体的新想法。获得分子铁电材料的关键是利用一维(LD)体系的强pi-电子-晶格相互作用特性，在有限温度(T_C)下促进相变到极性结构。这里的“晶格”指的是，例如，在S=1/2自旋的电荷转移(CT)化合物中的分子堆栈，显示平面-非平面跃迁的分子部分和pi-分子之间的氢键链。我们研究了两个典型的例子，这两个例子显示了这种一维铁电性的动态磁畴壁；(1)离子给体(D)-受体(A)电荷转移化合物，具有键有序波(BOW)型堆积二聚反应；(2)氢键固体，由-C=O和=C-OH基团的烯醇式分子组成。在前者中，自旋-晶格相互作用导致极弓态(D^+A^-或A^-D^+)，显示出T^c以下的三维有序性。在氢键pi分子固体中，分子内的键变化模式由相邻分子之间的质子位置(O=C-C=C-OH)决定。或HO-C=C-C=O..HO-C=C-C=O..HO-C=C-C=O.)。结果表明，在两种情况下，具有相反pi电子极化的两种简并结构之间确实存在可移动的扭结型畴壁(或孤子)。我们已经获得的实验结果表明，这种电荷和/或自旋载流畴状壁在这些新介质的介电和光电性能中起着至关重要的作用。

In this study, we have proposed new ideas for organic pi-molecular solids with low-dimensional ferroelectricity. The key to get the molecular ferroelectrics is to utilize the strong pi-electron-lattice interaction characteristic of the one-dimensional(lD) system which can promote the phase transition to the polar structure below finite temperature (T_c). Here, the "lattice" means , for example, the molecular stack in the charge-transfer(CT) compounds with s=1/2 spins, the molecular moiety showing the planar-nonplanar transition and the hydrogen-bonding chains between pi-molecules. We have studied two typical examples which show such a 1D ferroelectricity with dynamical domain-walls; (1) ionic donor(D)-acceptor(A) charge-transfer compounds with the bond-ordered-wave (BOW) type stack dimerization and (2) hydrogen bonded solids consisting of enol-form -molecules with the -C=O and =C-OH groups.In the former systems, the spin-lattice interaction causes the polar BOW states (D^+A^-D^+A^-D^+A^- or A^-D^+A^-D^+A^-D^+), which show the 3D ordering below T^c. On the other hand, in the hydrogen bonded pi- molecular solids the bond alternation pattern within the molecule is governed by the position of proton in-between the neighboring molecules (O=C-C=C-OH..O=C-C=C-OH..O=C-C=C-OH.. or HO-C=C-C=O..HO-C=C-C=O..HO-C=C-C=OH..). It was shown in both cases, the mobile kink-type domain-walls (or solitons) do exist between the two degenerate structures with opposite pi-electron polarization. We have obtained experimental results indicating such charge- and/or spin-carrying domain-walls play a crucial role in dielectric and photo-electric properties in those new dielectrics.