Fragile-to-strong transition in liquid silica

TL;DR

This study uses molecular dynamics simulations to identify a fragile-to-strong transition in liquid silica around 3100-3300 K, characterized by structural, density, and dynamical changes.

Contribution

It provides the first comprehensive evidence of a fragile-to-strong transition in liquid silica through multiple structural and dynamical indicators.

Findings

Density minimum at ~3000 K and maximum at 4700 K

Change from Arrhenius to Vogel-Fulcher-Tammann behavior around 3300 K

Stokes-Einstein relation breaks down above 3000 K

Abstract

We investigate anomalies in liquid silica with molecular dynamics simulations and present evidence for a fragile-to-strong transition at around 3100 K-3300 K. To this purpose, we studied the structure and dynamical properties of silica over a wide temperature range, finding four indicators of a fragile-to-strong transition. First, there is a density minimum at around 3000 K and a density maximum at 4700 K. The turning point is at 3400 K. Second, the local structure characterized by the tetrahedral order parameter changes dramatically around 3000 K from a higher-ordered, lower-density phase to a less ordered, higher-density phase. Third, the correlation time {\tau} changes from an Arrhenius behavior below 3300 K to a Vogel-Fulcher-Tammann behavior at higher temperatures. Fourth, the Stokes-Einstein relation holds for temperatures below 3000 K, but is replaced by a fractional relation above this temperature. Furthermore, our data indicate that dynamics become again simple above 5000 K, with Arrhenius behavior and a classical Stokes-Einstein relation.