Department of Physics, University of Michigan, Ann Arbor, Michigan 48109, USA.
Soft Matter. 2019 Mar 20;15(12):2571-2579. doi: 10.1039/c9sm00016j.
Orientational ordering is a necessary step in the crystallization of molecules and anisotropic colloids. Plastic crystals, which are possible mesophases between the fluid and fully ordered crystal, are translationally ordered but exhibit no long range orientational order. Here, we study the two-dimensional phase behavior of hard regular polygons with edge number n = 3-12. This family of particles provides a model system to isolate the effect of shape and symmetry on the existence of plastic crystal phases. We show that the symmetry group of the particle, G, and the symmetry group of the local environment in the crystal, H, together determine plastic colloidal crystal phase behavior in two dimensions. If G contains completely the symmetry elements of H, then a plastic crystal phase is absent. If G and H share some but not all nontrivial symmetry elements, then a plastic crystal phase exists with preferred particle orientations that recover the absent symmetry elements of the crystal; we call this phase the discrete plastic crystal phase. If G and H share no nontrivial symmetry elements, then a plastic crystal phase exists without preferred orientations, which we call an indiscrete plastic crystal.
取向有序是分子和各向异性胶体结晶的必要步骤。塑料晶体是介于流体和完全有序晶体之间的可能中间相，它在平移方向上有序，但没有长程取向有序。在这里，我们研究了边数 n = 3-12 的硬规则多边形的二维相行为。该粒子族提供了一个模型系统，可用于分离形状和对称性对塑料晶体相存在的影响。我们表明，粒子的对称群 G 和晶体中局部环境的对称群 H 共同决定了二维胶体的塑料晶体相行为。如果 G 完全包含 H 的对称元素，则不存在塑料晶体相。如果 G 和 H 共享一些但不是所有非平凡的对称元素，则存在具有恢复晶体中缺失对称元素的优先粒子取向的塑料晶体相；我们称这种相为离散塑料晶体相。如果 G 和 H 没有共享任何非平凡的对称元素，则存在没有优先取向的塑料晶体相，我们称之为无定形塑料晶体。
