Ferroelectric Molecular Crystals of Trichloroacetamides

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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