Selection mechanism of polymorphs in the crystal nucleation of the Gaussian core model
Selection mechanism of polymorphs in the crystal nucleation of the Gaussian core model
复制标题
DOI：
10.1039/c2sm07007c
复制
发表时间：
2012-03
期刊：
影响因子：
3.4
通讯作者：
J. Russo;Hajime Tanaka
中科院分区：
文献类型：
--
作者：
J. Russo;Hajime Tanaka
晶体成核的多晶型选择原理是结晶的基本问题之一。最近我们发现，对于硬球，晶体多晶型已经通过亚稳态过冷状态下局部有利的堆积对称性进行选择。在这里，我们研究这种情况是否也适用于软球体。为此，我们通过蒙特卡罗计算机模拟研究了过冷状态下高斯核心模型（GCM）的均匀成核过程。我们使用表征局部堆积对称性的键取向有序参数来跟踪固体核的形成并区分不同的多晶型物。我们专注于分别在低压和高压下的两个状态点，宏观热力学决定了不同多晶型物（分别为面心立方和面心立方晶体）的形成。我们表明，不同晶体结构的成核并不遵循奥斯特瓦尔德结晶阶跃规则，并且尽管存在底层相图，但 bcc 相始终受到青睐。与硬球系统类似，我们发现了多晶型选择的新标准：结晶发生在高键取向有序的前体区域，并且首先成核的晶体是在过冷状态下与这些有序区域最接近对称性的晶体。
The principle of polymorph selection upon crystal nucleation is one of the fundamental problems in crystallization. Recently we found that for hard spheres the crystal polymorph is already selected by locally favoured packing symmetry in a metastable supercooled state. Here we study whether this scenario is also valid for soft spheres. To do so, we investigate the homogeneous nucleation process of the Gaussian core model (GCM) in supercooled states by means of Monte Carlo computer simulations. We use bond orientational order parameters, which characterize local packing symmetries, to follow the formation of solid nuclei and to distinguish between different polymorphs. We concentrate on two state points, at low and high pressure respectively, for which macroscopic thermodynamics dictates the formation of the different polymorphs (fcc and bcc crystals respectively). We show that the nucleation of the different crystalline structures does not follow Ostwald's step rule of crystallization, and that, despite the underlying phase diagram, the bcc phase is always favoured. In analogy to hard sphere systems, we find a new criterion for polymorph selection: crystallization occurs in precursor regions of high bond orientational order, and the crystal which first nucleates is the one that has the closest symmetry to these ordered regions in the supercooled state.