Preparation of a cooling device for a Precession camera and study on a structural phase transition of a thiourea inclusion compound

一种以CBr_4为客体分子的硫脲包合物对分子间相互作用与物理性质之间的关系很感兴趣。Matsuo等人的热力学测量表明，该化合物在201.75 K时经历了一个相变，估计为有序-无序型。我拿起化合物来阐明由于相变引起的晶体结构的差异，并参考吉村的论文制作了进动相机的冷却装置，用x射线摄影技术对低温相进行晶体学研究。高温形式的（CH_4N_2S） _3 CBr_4在三角空间群R-3c中结晶。硫脲分子间氢键形成的通道呈蜂窝状结构，CBr_4在通道内占据特殊的32位，具有六重无序结构。假设硫脲分子所形成的蜂窝状结构与高温形态相比变化不大，仅改变了CBr_4的取向。低温形态的衍射斑出现轻微的劈裂，在相变点以上随着温度的升高而消失。在检查了各种双晶格后，对进差照片中的分裂点给出了满意的索引，该化合物被发现是一个共享ac面的双胞胎。它属于唯一轴c的单斜系统，具有γ ^*=62‰，可能的空间群为P2， Pm和P2/m。然而，平行于c轴的2重轴或垂直于c轴的镜面，使得即使采用任何空间群，也难以构建硫脲通道。因此，认为该晶体不属于单斜晶系，而属于三斜晶系。

A thiourea inclusion compound having guest molecules of CBr_4 is of interest with respect to the relationship between intermolecular interaction and physical properties. The thermodynamical measurements by Matsuo et al. indicated that the compound undergoes a phase transition estimated to be a order-disorder type at 201.75 K.I took up the compound to elucidate the difference of the crystal structures due to the phase transition, and made a cooling device for a Precession camera refering to Yoshimura's paper, to investigate crystallographycally the low temperature phase by X-ray photograph techniques. A high temperature form of (CH_4N_2S) _3 CBr_4 crystallizes in the trigonal space group R-3c. Channels constructed by hydrogen bondings between thiourea molecules adopt a honey-comb structure and CBr_4 occupies a special 32 position within the channel having a six-fold disorder structure.It seems reasonable to assume that the honey-comb structure constructed by thiourea molecules is little changed from the configuration in the high temperature form and only the orientation of CBr_4 is changed owing to the phase transition. Slight splittings appeared in the diffraction spots of the low temperature form and disappeared with an increase in temperature above the phase transition point. After various twin lattices were examined to give a satisfactory indexing for splitting spots in the Precession photographs, the compound was found to be a twin sharing an ac face. It belongs to the monoclinic system of unique axis c having gamma^*=62ﾟ, and the possible space groups are P2, Pm and P2/m. However, a 2-fold axis parallel to c axis or a mirror plane perpendicular to one makes it difficult to construct the channels of thiourea, even if any space group is adopted. It is, therefore, considered that the crystal does not belong to the monoclinic but to the triclinic system.