Ferroelectric Molecular Crystals of Trichloroacetamides

为了研究三氯乙酰胺（TCAA）在355 K铁电III-II转变的机理，测定了TCAA的III相（300 K X射线衍射法，90、300和343 K粉末中子衍射法）、II相（357 K单晶中子衍射法）和I相（370 K粉末中子衍射法）的晶体结构。各相的空间群均为P2_1。在TCAA的第三阶段中，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点，这是由围绕C-C轴的小角度旋转产生的，并且在高温阶段中，发现它占据对应于两个位点的平均值的位置。晶体结构（中子衍射法）表明，其中一个氨基的结构随温度的变化显着。对红外谱带（N-H伸缩和NH_2面外振动）的温度依赖性的精确观测表明， ...更多信息 部分谱带的波数随温度变化显著，在转变温度附近出现反常位移。TCAA的铁电转变可能是由于三氯甲基在两个位置之间的小角度旋转和氨基结构的变化共同作用的结果.三溴乙酰胺（TBAA）的室温相也呈现出类似于TCAA的两个位置结构.然而，实验结果表明，TCAA的铁电转变可能是由于三氯甲基在两个位置之间的小角度旋转和氨基结构的变化共同作用的结果. 2-溴-2，2-二氯乙酰胺（BDCAA）和2-氯-2，2-二溴乙酰胺（CDBAA）的相变与TCAA的相变非常相似，它们可能是铁电体。介电常数变化幅度的降低归因于钉扎效应，本文提出了纯分子晶体的铁电性可能是由有机化合物特有的内旋转自由和分子结构的部分变形引起的。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