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

Abstract

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

Pattern of property extrema in supercooled and stretched water models and a new correlation for predicting the stability limit of the liquid state

“…The thermodynamic stability limit is not directly attainable, but there are theoretical studies that investigate the relationship between the kinetic and thermodynamic stability limits of water, 9,22,71 and future work could utilize these studies to investigate the liquid-vapor spinodal for real water. It will be also interesting to explore the extent to which the present scaling procedure can be successfully applied to other tetrahedral systems exhibiting water-like behavior, such as tetrahedral patchy colloids, 72 silicon, [29][30][31] and silica. 32 We plan to pursue such studies in the future.…”

“…22,23 One possible explanation for water's anomalous behavior is the existence of a metastable first-order phase transition involving two distinct liquid phases that terminates at a liquidliquid critical point (LLCP). 5,18 This liquid-liquid transition is considered to be a special case of fluid polyamorphism, 25 a phenomenon that has been either found or hypothesized in a broad range of materials including metallic hydrogen, [26][27][28] silicon, [29][30][31] silica, 32,33 carbon, 34 cerium, 35 and phosporous. 36 Although experimental evidence consistent with polyamorphism exists for supercooled water, 6,37 a definitive proof is still lacking.…”

“…Computer simulations, on the other hand, have shown that some molecular models of water exhibit liquid-liquid separation at deeply supercooled conditions. 22,[38][39][40][41][42][43][44][45][46][47][48] In tetrahedral systems, regardless of the existence or non-existence of the liquid-liquid separation (e.g., mW, [49][50][51] mTIP4P, 50 ST2, 52 The Journal of Chemical Physics ARTICLE scitation.org/journal/jcp TIP4P/2005, 53 TIP5P, 54 silicon, 30,31,55 silica, 32,33 and germanium 56 ), there is an underlying characteristic pattern of extrema lines for thermodynamic properties. The most wellknown is the line of density extrema, whose existence suggests a competition between low-density and high-density structures in the same liquid.…”

The interactions between thermodynamic anomalies

“…and we can obtain the final expression for the gradient by inserting Eqs. (27) and (25) into the expression for the gradient of the compressibility anomaly,…”

“…(SI-48) and Eq. (27), onto which we also apply the heat capacity anomaly condition, into the above equation to obtain…”