Parkin S R
Department of Chemistry, University of Kentucky, Lexington, KY 40506, USA.
Acta Crystallogr E Crystallogr Commun. 2021 Apr 9;77(Pt 5):452-465. doi: 10.1107/S205698902100342X. eCollection 2021 May 1.
Twinning by pseudo-merohedry is a common phenomenon in small-mol-ecule crystallography. In cases where twin-component volume fractions are markedly different, structure solution is often no more difficult than for non-twinned structures of similar complexity. When twin-component volume fractions are similar, however, structure solution can be much more of a problem. This paper presents hints and tips for such cases by means of three worked examples. The first example presents the most common (and simplest) case of a two-component pseudo-ortho-rhom-bic twin. The second example describes structure solution of a reticular threefold pseudo-hexa-gonal twin that benefits from use of an unconventional space-group setting. The third example covers structure solution of a reticular fourfold pseudo-tetra-gonal twin. All three structures are ultimately shown to be monoclinic crystals that twin as a consequence of unit-cell metrics that mimic those of higher symmetry crystal systems.
通过假等轴面孪晶是小分子晶体学中的常见现象。在孪晶组分体积分数明显不同的情况下，结构解析通常并不比解析具有相似复杂性的非孪晶结构困难。然而，当孪晶组分体积分数相似时，结构解析可能会成为一个更大的问题。本文通过三个实例介绍了针对此类情况的提示和技巧。第一个实例展示了两组分假正交斜方孪晶最常见（也是最简单）的情况。第二个实例描述了网状三重假六方孪晶的结构解析，该解析受益于使用非常规的空间群设置。第三个实例涵盖了网状四重假四方孪晶的结构解析。最终表明，所有这三种结构都是单斜晶体，由于其晶胞参数模仿了更高对称晶体系统的参数而发生孪晶。
