Why Are Some Crystals Straight?
Why Are Some Crystals Straight?
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DOI：
10.1021/acs.jpcc.0c04258
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发表时间：
2020-07-16
影响因子：
3.7
通讯作者：
Kahr, Bart
中科院分区：
文献类型：
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作者：
Li, Chao;Shtukenberg, Alexander G.;Kahr, Bart
超过四分之一的能够熔化的分子晶体可以以扭曲的片状或纤维的形式生长。导致这种不寻常的晶体形态在中观尺度上缺乏长程平移对称性的机制还知之甚少。苯并(C6H5C(O)-C(O)-C6H5)就是这样一种晶体。在这里，我们计算了棒状苯并纳米晶和其他相关结构的形貌。对于横截面为50-10 nm(2)的棒，这些系综的基态分别扭曲了0.05-0.75度/埃；扭曲程度与晶体横截面积成反比。总的来说，我们的计算研究，结合早期的光学显微镜观察，表明在某些情况下，非常小的晶体只有在达到一定尺寸后才能获得3D平移周期。扭曲伴随着六边杆的{10(1)上bar0}面上分子的构象变化，尽管从我们的数据中很难回答这种变化是扭曲的原因，对称性被破坏的表面应力的结果，还是当两个或更多几何倾向冲突时内在不对称的结果。然而，在某些情况下，分子聚集体的热力学并不能预测具有长程平移对称性晶格的晶体的发展。相反，晶格有时是一种允许正在生长的晶体利用生长的热力学驱动力的装置，对于大量分子来说，这是最好的折衷方案，在较小的尺度上，分子是不对称的(只有点对称)。讨论了这些计算与介观尺度上普遍存在的晶体扭曲之间的关系。
More than one-quarter of molecular crystals that are able to be melted can be made to grow in the form of twisted lamellae or fibers. The mechanisms leading to such unusual crystal morphologies lacking long-range translational symmetry on the mesoscale are poorly understood. Benzil (C6H5C(O)-C(O)-C6H5) is one such crystal. Here, we calculate the morphology of rod-shaped benzil nanocrystals and other related structures. The ground states of these ensembles were twisted by 0.05-0.75 degrees /angstrom for rods with cross sections of 50-10 nm(2), respectively; the degree of twisting decreased inversely proportional to the crystal cross-sectional area. In the aggregate, our computational studies, combined with earlier observations by light microscopy, suggest that in some cases very small crystals acquire 3D translational periodicity only after reaching a certain size. Twisting is accompanied by conformational changes of molecules on the {10 (1) over bar0} surfaces of the six-sided rods, although it is not easily answered from our data whether such changes are causes of the twisting, consequences of surface stress where symmetry is broken, or consequences of intrinsic dissymmetry when two or more geometric tendencies are in conflict. Nevertheless, it has become dear that, in some cases, the development of a crystal with a lattice having long-range translational symmetry is not foretold in the thermodynamics of aggregates of molecules. Rather, a lattice is sometimes a device for allowing a growing crystal to take advantage of the thermodynamic driving force of growth, the best compromise for a large number of molecules, which on a smaller scale would be dissymmetric (have a point symmetry only). The relationship between these calculations and the ubiquity of crystal twisting on the mesoscale are discussed.