Department of Chemistry, University of Utah, Salt Lake City, UT 84112-0850, USA.
Phys Chem Chem Phys. 2011 Nov 28;13(44):20008-16. doi: 10.1039/c1cp22022e. Epub 2011 Oct 19.
Ice crystallized below 200 K has the diffraction pattern of a faulty cubic ice, and not of the most stable hexagonal ice polymorph. The origin and structure of this faulty cubic ice, presumed to form in the atmosphere, has long been a puzzle. Here we use large-scale molecular dynamics simulations with the mW water model to investigate the crystallization of water at 180 K and elucidate the development of cubic and hexagonal features in ice as it nucleates, grows and consolidates into crystallites with characteristic dimensions of a few nanometres. The simulations indicate that the ice crystallized at 180 K contains layers of cubic ice and hexagonal ice in a ratio of approximately 2 to 1. The stacks of hexagonal ice are very short, mostly one and two layers, and their frequency does not seem to follow a regular pattern. In spite of the high fraction of hexagonal layers, the diffraction pattern of the crystals is, as in the experiments, almost identical to that of cubic ice. Stacking of cubic and hexagonal layers is observed for ice nuclei with as little as 200 water molecules, but a preference for cubic ice is already well developed in ice nuclei one order of magnitude smaller: the critical ice nuclei at 180 K contain approximately ten water molecules in their core and are already rich in cubic ice. The energies of the cubic-rich and hexagonal-rich nuclei are indistinguishable, suggesting that the enrichment in cubic ice does not have a thermodynamic origin.
在 200 K 以下结晶的冰具有结构缺陷的立方冰的衍射图案，而不是最稳定的六方冰多晶型物。这种结构缺陷的立方冰的起源和结构，据推测是在大气中形成的，长期以来一直是一个谜。在这里，我们使用具有 mW 水模型的大规模分子动力学模拟来研究水在 180 K 下的结晶，并阐明在冰成核、生长和凝聚成具有几个纳米特征尺寸的微晶时，立方和六方特征的发展。模拟表明，在 180 K 下结晶的冰包含立方冰和六方冰的层，比例约为 2 比 1。六方冰的堆积非常短，大多只有一层和两层，其频率似乎没有遵循规则模式。尽管六方层的比例很高，但晶体的衍射图案与实验中的立方冰几乎相同。具有少至 200 个水分子的冰核就可以观察到立方和六方层的堆积，但在小一个数量级的冰核中，立方冰的优先性已经得到很好的发展：在 180 K 下的临界冰核核心大约含有 10 个水分子，并且已经富含立方冰。具有丰富立方冰的核和具有丰富六方冰的核的能量是不可区分的，这表明立方冰的富集没有热力学起源。
