Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India.
Department of Chemistry, The University of Utah, Salt Lake City, Utah 84112-0850, USA.
J Chem Phys. 2016 Dec 7;145(21):214502. doi: 10.1063/1.4967939.
We use molecular dynamics simulations to compare and contrast the liquid-state anomalies in the Stillinger-Weber models of monatomic water (mW), silicon (Si), and germanium (Ge) over a fairly wide range of temperatures and densities. The relationships between structure, entropy, and mobility, as well as the extent of the regions of anomalous behavior, are discussed as a function of the degree of tetrahedrality. We map out the cascade of density, structural, pair entropy, excess entropy, viscosity, and diffusivity anomalies for these three liquids. Among the three liquids studied here, only mW displays anomalies in the thermal conductivity, and this anomaly is evident only at very low temperatures. Diffusivity and viscosity, on the other hand, show pronounced anomalous regions for the three liquids. The temperature of maximum density of the three liquids shows re-entrant behavior consistent with either singularity-free or liquid-liquid critical point scenarios proposed to explain thermodynamic anomalies. The order-map, which shows the evolution of translational versus tetrahedral order in liquids, is different for Ge than for Si and mW. We find that although the monatomic water reproduces several thermodynamic and dynamic properties of rigid-body water models (e.g., SPC/E, TIP4P/2005), its sequence of anomalies follows, the same as Si and Ge, the silica-like hierarchy: the region of dynamic (diffusivity and viscosity) anomalies encloses the region of structural anomalies, which in turn encloses the region of density anomaly. The hierarchy of the anomalies based on excess entropy and Rosenfeld scaling, on the other hand, reverses the order of the structural and dynamic anomalies, i.e., predicts that the three Stillinger-Weber liquids follow a water-like hierarchy of anomalies. We investigate the scaling of diffusivity, viscosity, and thermal conductivity with the excess entropy of the liquid and find that for dynamical properties that present anomalies there is no universal scaling of the reduced property with excess entropy for the whole range of temperatures and densities. Instead, Rosenfeld's scaling holds for all the three liquids at high densities and high temperatures, although deviations from simple exponential dependence are observed for diffusivity and viscosity at lower temperatures and intermediate densities. The slope of the scaling of transport properties obtained for Ge is comparable to that obtained for simple liquids, suggesting that this low tetrahedrality liquid, although it stabilizes a diamond crystal, is already close to simple liquid behavior for certain properties.
我们使用分子动力学模拟来比较和对比单原子水 (mW)、硅 (Si) 和锗 (Ge) 的 Stillinger-Weber 模型在相当宽的温度和密度范围内的液态异常。讨论了结构、熵和流动性之间的关系，以及异常行为区域的程度，作为四面体程度的函数。我们绘制了这三种液体的密度、结构、对熵、过剩熵、粘度和扩散率异常的级联。在研究的三种液体中，只有 mW 在热导率方面表现出异常，并且这种异常仅在非常低的温度下明显。另一方面，扩散率和粘度对于三种液体都显示出明显的异常区域。三种液体的最大密度温度表现出与提出的解释热力学异常的无奇异或液-液相变临界点方案一致的再进入行为。顺序图显示了液体中平移与四面体形序的演化，对于 Ge 与 Si 和 mW 不同。我们发现，尽管单原子水再现了刚体水模型（例如 SPC/E、TIP4P/2005）的几种热力学和动力学性质，但它的异常序列与 Si 和 Ge 相同，遵循硅样层次结构：动态（扩散率和粘度）异常区域包围结构异常区域，结构异常区域又包围密度异常区域。另一方面，基于过剩熵和罗森菲尔德标度的异常层次结构反转了结构和动力学异常的顺序，即预测这三种 Stillinger-Weber 液体遵循水样异常层次结构。我们研究了扩散率、粘度和热导率与液体过剩熵的标度，并发现对于呈现异常的动力学性质，在整个温度和密度范围内，减少的性质与过剩熵没有普遍的标度。相反，罗森菲尔德标度适用于所有三种液体在高密度和高温下，尽管在较低温度和中等密度下观察到扩散率和粘度的简单指数依赖性的偏差。对于 Ge 获得的输运性质的标度斜率与简单液体获得的斜率相当，这表明这种低四面体液体尽管稳定了金刚石晶体，但对于某些性质已经接近简单液体行为。
