School of Physics and Optoelectronic Engineering, Ludong University, Yantai 264025, P. R. China.
Sci Rep. 2017 Jan 5;7:39952. doi: 10.1038/srep39952.
The relationship between structural order and water-like anomalies in tetrahedral liquids is still open. Here, first-principle molecular dynamics are performed to study it in metastable liquid Si. It is found that in T-P phase diagram, there indeed exists a structural anomaly region, which encloses density anomaly but not diffusivity anomaly. This is consistent with that of SW Si and BKS SiO but different from that of SPC/E water. Two-body excess entropy anomaly can neither capture the diffusivity, structural, and density anomalies, as it can in a two-scale potential fluid. In structural anomaly region, tetrahedrality order q (measuring the extent to which an atom and its four nearest neighbours adopt tetrahedral arrangement) and translational order t (measuring the tendency of two atoms to adopt preferential separation) are not perfectly correlated, which is different from that in SW Si and renders it impossible to use the isotaxis line to quantify the degree of structural order needed for water-like anomalies to occur. Along the isotherm of critical temperature T, t/q is approximately linear with pressure. With decreasing pressure along the isotherm below T, t/q departs downward from the line, while it is the opposite case above T.
四面体液体中结构有序性和类水异常之间的关系仍未得到解决。在这里，我们通过第一性原理分子动力学方法对亚稳液态硅进行了研究。结果发现，在 T-P 相图中，确实存在一个结构异常区域，它包围了密度异常，但不包括扩散率异常。这与 SW Si 和 BKS SiO 一致，但与 SPC/E 水不同。双体过剩熵异常既不能像在两尺度势流体中那样捕捉到扩散率、结构和密度异常。在结构异常区域中，四面体有序度 q（衡量一个原子与其四个最近邻原子采用四面体排列的程度）和平移有序度 t（衡量两个原子采用优先分离的趋势）之间没有完全相关，这与 SW Si 不同，因此无法使用同晶线来量化发生类水异常所需的结构有序度。沿着临界温度 T 的等温线，t/q 与压力近似呈线性关系。沿着等温线在 T 以下降低压力时，t/q 从线上向下偏离，而在 T 以上则相反。
