Ferroelectric Molecular Crystals of Trichloroacetamides

为了研究三氯乙酰胺(TCAA)在355K发生铁电III-II转变的机理，我们测定了相III(300K X射线衍射法，90、300和343K粉末中子衍射法)、II相(357K单晶中子衍射法)和I相(370K粉末中子衍射法)的晶体结构。各相的空间群为P2_1。在TCAA的第三阶段，发现两个独立的三氯甲基中的一个能够占据两个不同的位置，这两个位置是围绕C-C轴小角旋转产生的，而在高温阶段，它占据的位置相当于两个位置的平均值。晶体结构(中子衍射法)表明，其中一个氨基的结构随温度发生显著变化。对红外光谱(N-H伸缩和NH_2出平面振动)随温度变化的精确观测表明，…部分谱带的较多波数随温度变化明显，并在转变温度附近表现出反常的位移。作为TCAA铁电转变的一种可能机制，我们提出了三氯甲基在两个中心之间的小角度旋转和氨基结构变化的结合。三溴乙酰胺(TBAA)的室温相也呈现出与TCAA相似的两个中心结构。然而，实验结果表明，TBAA的相变机理与TCAA完全不同。2-溴-2，2-二氯乙酰胺(BDCAA)和2-氯-2，2-二溴乙酰胺(CDBAA)的相变很可能是铁电的。介电常数随转变而变化幅度的降低归因于钉扎效应。本工作提出了一种纯分子晶体铁电性的可能性，这是由于有机化合物的内旋转自由和分子结构的部分变形造成的。较少

In order to investigate the mechanism of the ferroelectric III-II transition of trichloroacetamide (TCAA) at 355 K,we determined the crystal structures of phase III (X-ray diffraction method at 300 K,powder neutron diffraction method at 90,300 and 343 K), phase II (single crystal neutron diffraction method at 357 K) and phase I (powder neutron diffraction method at 370 K). The space group of each phase was P2_1. In Phase III of TCAA,one of the two independent trichloromethyl groups was found to be able to occupy two different sites which are produced by a small angle rotation around the C-C axis and in the high temperature phase it was found to occupy a position which corresponds to an average of the two sites. The crystal structure (neutron diffraction method) showed that the structure of one of the amino groups changed significantly with temperature. Precise observations of the temperature dependence of the IR bands (N-H stretching and NH_2 out of plane vibrations) revealed that the … More wavenumber of a part of these bands changed remarkably depending on the temperature and showed anomalous shifted in the vicinity of the transition temperatures. As a possible mechanism of the ferroelectric transition of TCAA we proposed a combination of the small angle rotation of the trichloromethyl group between the two-sites and the change in the structure of the amino group.The room temperature phase of tribromoacetamide (TBAA) also exhibits a two-sites structure similar to that of TCAA.Experimental results indicated, however, that the mechanism of the phase transition in TBAA was completely different from that of TCAA.2-bromo-2,2-dichloroacetamide (BDCAA) and 2-chloro-2,2-dibromoacetamide (CDBAA) showed phase transitions quite similar to those of TCAA and they are probably ferroelectric. The depression of the magnitude of the change in the dielectric constant accompanied by the transition was atttributed to a pin-effect.The present work sugested a possibility of ferroelectricity of pure molecular crystals caused by a freedom of internal rotations characteristic of organic compounds and partial deformations of the mplecular structure. Less