Department of Physics, Brooklyn College of the City University of New York, Brooklyn, New York 11210, USA.
J Chem Phys. 2013 Nov 14;139(18):184504. doi: 10.1063/1.4829276.
Glassy water occurs in at least two broad categories: low-density amorphous (LDA) and high-density amorphous (HDA) solid water. We perform out-of-equilibrium molecular dynamics simulations to study the transformations of glassy water using the ST2 model. Specifically, we study the known (i) compression-induced LDA-to-HDA, (ii) decompression-induced HDA-to-LDA, and (iii) compression-induced hexagonal ice-to-HDA transformations. We study each transformation for a broad range of compression/decompression temperatures, enabling us to construct a "P-T phase diagram" for glassy water. The resulting phase diagram shows the same qualitative features reported from experiments. While many simulations have probed the liquid-state phase behavior, comparatively little work has examined the transitions of glassy water. We examine how the glass transformations relate to the (first-order) liquid-liquid phase transition previously reported for this model. Specifically, our results support the hypothesis that the liquid-liquid spinodal lines, between a low-density and high-density liquid, are extensions of the LDA-HDA transformation lines in the limit of slow compression. Extending decompression runs to negative pressures, we locate the sublimation lines for both LDA and hyperquenched glassy water (HGW), and find that HGW is relatively more stable to the vapor. Additionally, we observe spontaneous crystallization of HDA at high pressure to ice VII. Experiments have also seen crystallization of HDA, but to ice XII. Finally, we contrast the structure of LDA and HDA for the ST2 model with experiments. We find that while the radial distribution functions (RDFs) of LDA are similar to those observed in experiments, considerable differences exist between the HDA RDFs of ST2 water and experiment. The differences in HDA structure, as well as the formation of ice VII (a tetrahedral crystal), are a consequence of ST2 overemphasizing the tetrahedral character of water.
低密度非晶态（LDA）和高密度非晶态（HDA）固体水。我们使用 ST2 模型进行非平衡分子动力学模拟，以研究玻璃态水的转变。具体来说，我们研究了已知的（i）压缩诱导的 LDA 到 HDA，（ii）减压诱导的 HDA 到 LDA，和（iii）压缩诱导的六方冰到 HDA 的转变。我们在广泛的压缩/减压温度范围内研究每种转变，从而为玻璃态水构建了一个“P-T 相图”。所得的相图显示出与实验报告的相同定性特征。虽然许多模拟已经探测了液态相行为，但相对较少的工作研究了玻璃态水的转变。我们研究了玻璃转变如何与先前报道的该模型的（一级）液-液相转变相关。具体来说，我们的结果支持了这样的假设，即在低密度和高密度液体之间的液-液旋节线是 LDA-HDA 转变线在缓慢压缩极限下的延伸。通过将减压运行扩展到负压，我们定位了 LDA 和超淬火玻璃态水（HGW）的升华线，并发现 HGW 相对更稳定于蒸汽。此外，我们在高压下观察到 HDA 的自发结晶到冰 VII。实验也观察到 HDA 的结晶，但结晶到冰 XII。最后，我们将 ST2 模型的 LDA 和 HDA 的结构与实验进行对比。我们发现，虽然 LDA 的径向分布函数（RDFs）与实验观察到的相似，但 ST2 水的 HDA RDFs 与实验之间存在相当大的差异。HDA 结构的差异以及冰 VII（四面体晶体）的形成是 ST2 过分强调水的四面体特征的结果。
