Molecular structural order and anomalies in liquid silica

TL;DR

This study uses molecular dynamics simulations to explore the relationship between structural order, diffusivity anomalies, and density anomalies in liquid silica, revealing differences from water in how order parameters behave under compression.

Contribution

It provides new insights into the structural and thermodynamic anomalies of liquid silica, highlighting differences from water in the coupling of order parameters and anomalies.

Findings

Silica shows bimodal distribution of local orientational order at intermediate densities.

Order parameter extrema occur upon compression, with a broad minimum in translational order.

Structural and diffusivity anomalies in silica are not fully coupled, unlike in water.

Abstract

The present investigation examines the relationship between structural order, diffusivity anomalies, and density anomalies in liquid silica by means of molecular dynamics simulations. We use previously defined orientational and translational order parameters to quantify local structural order in atomic configurations. Extensive simulations are performed at different state points to measure structural order, diffusivity, and thermodynamic properties. It is found that silica shares many trends recently reported for water [J. R. Errington and P. G. Debenedetti, Nature 409, 318 (2001)]. At intermediate densities, the distribution of local orientational order is bimodal. At fixed temperature, order parameter extrema occur upon compression: a maximum in orientational order followed by a minimum in translational order. Unlike water, however, silica's translational order parameter minimum is broad, and there is no range of thermodynamic conditions where both parameters are strictly coupled. Furthermore, the temperature-density regime where both structural order parameters decrease upon isothermal compression (the structurally anomalous regime) does not encompass the region of diffusivity anomalies, as was the case for water.