Shi Rui, Tanaka Hajime
Department of Fundamental Engineering, Institute of Industrial Science, University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan.
J Chem Phys. 2019 Jul 14;151(2):024501. doi: 10.1063/1.5100634.
Recently, it has been revealed that a supercooled liquid below the melting point has intrinsic structural heterogeneity due to local structural ordering as a manifestation of many-body correlations. The impact of such preordering on crystal nucleation has recently attracted considerable attention. In this work, by extensive molecular dynamics simulations of TIP5P water, we find a two-step homogeneous nucleation of a ferroelectric ice crystal: the first step is spontaneous dipolar ordering, i.e., paraelectric-to-ferroelectric transition, in a supercooled liquid state and the second step is the nucleation of the ferroelectric ice crystal selectively in the preordered regions. We reveal that in this system the dipole-dipole correlation grows rapidly with an increase in pressure, eventually leading to spontaneous dipolar ordering at a certain condition (e.g., at 2000 bars and 227 K). This result is obtained by simulations of TIP5P water with a simple cutoff of Coulomb interactions. By comparing this result with those of the particle-mesh Ewald and reaction field treatments of the Coulomb interactions, we find that the potential cutoff significantly enhances the dipole-dipole correlation, resulting in the fast ice nucleation to the ferroelectric cubic form. Despite the unrealistic enhancement of dipolar correlation in this model, this work provides an intriguing physical scenario of two-step crystal nucleation in polar molecules assisted by dipolar orientational ordering, which may be relevant to crystallizations, e.g., under an external electric field, on a charged surface, or under extreme conditions.
最近研究发现，熔点以下的过冷液体由于作为多体关联表现的局部结构有序性而具有内在结构不均匀性。这种预排序对晶体成核的影响最近引起了相当大的关注。在这项工作中，通过对TIP5P水进行广泛的分子动力学模拟，我们发现了铁电冰晶的两步均匀成核：第一步是在过冷液体状态下自发的偶极有序化，即从顺电到铁电的转变，第二步是在预排序区域中选择性地形成铁电冰晶。我们揭示，在该系统中，偶极-偶极相关性随压力增加而迅速增长，最终在特定条件下（例如，在2000巴和227K时）导致自发的偶极有序化。这一结果是通过对具有简单库仑相互作用截断的TIP5P水进行模拟得到的。通过将这一结果与对库仑相互作用采用粒子网格埃瓦尔德方法和反应场方法得到的结果进行比较，我们发现势截断显著增强了偶极-偶极相关性，导致快速形成铁电立方晶型的冰核。尽管该模型中偶极相关性的增强不切实际，但这项工作提供了一个有趣的物理场景，即极性分子中由偶极取向有序化辅助的两步晶体成核，这可能与例如在外部电场下、在带电表面上或在极端条件下的结晶过程相关。
