Zimoń Małgorzata J, Martelli Fausto
IBM Research Europe, Hartree Centre, Daresbury WA4 4AD, United Kingdom.
J Chem Phys. 2023 Mar 21;158(11):114501. doi: 10.1063/5.0138368.
We report a molecular dynamics study of the heterogeneous crystallization of high-pressure glassy water using (plastic) ice VII as a substrate. We focus on the thermodynamic conditions P ∈ [6-8] GPa and T ∈ [100-500] K, at which (plastic) ice VII and glassy water are supposed to coexist in several (exo)planets and icy moons. We find that (plastic) ice VII undergoes a martensitic phase transition to a (plastic) fcc crystal. Depending on the molecular rotational lifetime τ, we identify three rotational regimes: for τ > 20 ps, crystallization does not occur; for τ ∼ 15 ps, we observe a very sluggish crystallization and the formation of a considerable amount of icosahedral environments trapped in a highly defective crystal or in the residual glassy matrix; and for τ < 10 ps, crystallization takes place smoothly, resulting in an almost defect-free plastic fcc solid. The presence of icosahedral environments at intermediate τ is of particular interest as it shows that such a geometry, otherwise ephemeral at lower pressures, is, indeed, present in water. We justify the presence of icosahedral structures based on geometrical arguments. Our results represent the first study of heterogeneous crystallization occurring at thermodynamic conditions of relevance for planetary science and unveil the role of molecular rotations in achieving it. Our findings (i) show that the stability of plastic ice VII, widely reported in the literature, should be reconsidered in favor of plastic fcc, (ii) provide a rationale for the role of molecular rotations in achieving heterogeneous crystallization, and (iii) represent the first evidence of long-living icosahedral structures in water. Therefore, our work pushes forward our understanding of the properties of water.
我们报告了一项利用（塑性）冰VII作为基底对高压玻璃态水的非均相结晶进行的分子动力学研究。我们关注的热力学条件为P ∈ [6 - 8] GPa和T ∈ [100 - 500] K，在这些条件下，（塑性）冰VII和玻璃态水被认为共存于几颗（系外）行星和冰卫星中。我们发现（塑性）冰VII经历了马氏体相变成为（塑性）面心立方晶体。根据分子旋转寿命τ，我们确定了三种旋转状态：当τ > 20 ps时，不发生结晶；当τ ∼ 15 ps时，我们观察到结晶非常缓慢，并且形成了大量被困在高度缺陷晶体或残余玻璃态基质中的二十面体环境；当τ < 10 ps时，结晶顺利发生，形成几乎无缺陷的塑性面心立方固体。中间τ值时二十面体环境的存在特别令人感兴趣，因为这表明这种几何结构在较低压力下原本是短暂存在的，但在水中确实存在。我们基于几何论证解释了二十面体结构的存在。我们的结果代表了对在与行星科学相关的热力学条件下发生的非均相结晶的首次研究，并揭示了分子旋转在实现这一过程中的作用。我们的发现（i）表明，文献中广泛报道的塑性冰VII的稳定性应重新考虑，转而支持塑性面心立方；（ii）为分子旋转在实现非均相结晶中的作用提供了理论依据；（iii）代表了水中长寿命二十面体结构的首个证据。因此，我们的工作推动了我们对水的性质的理解。
