Gardon Manuel, Pinheiro Carlos B, Chapuis Gervais
Institut de Cristallographie, Université de Lausanne, BSP Dorigny, CH-1015 Lausanne, Switzerland.
Acta Crystallogr B. 2003 Aug;59(Pt 4):527-36. doi: 10.1107/s0108768103011297. Epub 2003 Jul 25.
The thermotropic phase diagram of 1:1 co-crystals of hexamethylenetetramine and pimelic acid (heptanedioic acid) is investigated. Three crystalline phases are identified at ambient pressure. Phase I is disordered, as revealed by diffuse rods in its diffraction pattern. When the temperature is lowered the diffuse streaks disappear in Phase II, but superstructure reflections emerge indicating an ordering process of the structure through a non-ferroic, or at least non-ferroelastic, phase transition. Phase II is mainly characterized by an unusual distribution of its reflection intensities. Phase III is reached through a ferroelastic phase transition that induces twinned domains. A model based on the stacking of an elementary layer is proposed with the aim of describing the structures in a unified framework. Depending on the value of the unique stacking parameter eta, each of the different structures observed can be reproduced by this model. Its validity is then tested by a series of simulations reproducing the main features of the diffraction patterns such as the diffuse scattering streaks, the occurrence of superstructure peaks at lower temperature and twinning.
研究了六亚甲基四胺与庚二酸1:1共晶体的热致相图。在常压下确定了三个晶相。相I是无序的，这在其衍射图中由漫散棒状结构显示出来。当温度降低时，漫散条纹在相II中消失，但超结构反射出现，表明通过非铁电或至少非铁弹性相变，结构发生了有序化过程。相II的主要特征是其反射强度分布异常。相III是通过诱导孪晶畴的铁弹性相变达到的。提出了一个基于基本层堆叠的模型，旨在在统一框架内描述这些结构。根据唯一堆叠参数η的值，该模型可以重现观察到的每种不同结构。然后通过一系列模拟来测试其有效性，这些模拟重现了衍射图的主要特征，如漫散射条纹、低温下超结构峰的出现和孪晶现象。
