Diversity of transition pathways in the course of crystallization into ice VII
Diversity of transition pathways in the course of crystallization into ice VII
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DOI：
10.1039/c4cp01616e
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发表时间：
2014-08-21
影响因子：
3.3
通讯作者：
Matsumoto, Masakazu
中科院分区：
文献类型：
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作者：
Mochizuki, Kenji;Himoto, Kazuhiro;Matsumoto, Masakazu
我们报道了在10 Gpa，425K下，通过大规模分子动力学(MD)模拟，在冻结成高压冰VII的过程中出现的各种类型的路径。根据Ostwald的台阶规则，一些轨迹显示出明显的通过独特的亚稳态“相”的阶跃转变。亚稳结构被鉴定为具有旋转相(塑性相)性质的四面体紧密堆积结构之一。该晶胞由21个水分子组成，这些水分子尚未被模拟或实验报道。对众多轨迹的结构分析表明，冰VII的成核很容易发生在亚稳态胚胎的晶界，这被称为外延介导的转变，符合Ostwald的阶跃规则。因此，晶界的大小决定了亚稳相的寿命。另一方面，一旦冰七的胚胎出现在液体中亚稳定相的胚胎之前，就会发生向冰七的直接转变。我们还发现，亚稳相的塑性行为并不是均匀的，而是对相邻分子的相互作用强度和局域结构敏感，因此我们称之为“部分塑性冰”。
We report various types of pathways emerging in the course of freezing into a high pressure ice VII by large-scale molecular dynamics (MD) simulations at 10 GPa, 425 K. Some trajectories showed an apparently stepwise transition via the unique metastable "phase" in accordance with Ostwald's step rule. The metastable structure was identified as one of the tetrahedrally close-packed structures having the nature of a rotator phase (plastic phase). The unit cell consists of 21 water molecules that has not yet been reported by simulation or by experiments. Structure analysis of numerous trajectories reveals that the nucleation of ice VII easily occurs at the grain boundaries of the metastable embryos, known as epitaxy-mediated transformation in line with Ostwald's step rule. The size of the grain boundaries is thus responsible for the lifetime of the metastable phase. On the other hand, once the embryo of ice VII appears prior to that of the metastable phase in liquid, direct transition to ice VII takes place. We also show that the plastic behavior of the metastable phase is not uniform but is sensitive to the interaction strength of adjacent molecules and the local structure, thus we call "partially plastic ice".