Relationship between structural order and water-like anomalies in metastable liquid silicon: Ab initio molecular...

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 SiO₂ 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_tetra (measuring the extent to which an atom and its four nearest neighbours adopt tetrahedral arrangement) and translational order t_trans (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_c, t_trans/q_tetra is approximately linear with pressure. With decreasing pressure along the isotherm below T_c, t_trans/q_tetra departs downward from the line, while it is the opposite case above T_c.