Understanding the fragile-to-strong transition in silica from microscopic dynamics

TL;DR

This study investigates the microscopic dynamics behind the fragile-to-strong transition in silica, revealing two distinct diffusion channels with different energy barriers that govern the transition and are linked to thermodynamic properties.

Contribution

It introduces a machine learning approach to analyze microscopic activation energetics, uncovering the origin of the FTS in silica and its connection to thermodynamics.

Findings

Two diffusion channels with different energy barriers identified.

Transition at around 3100 K correlates with entropy inflection.

Fast and slow dynamics dominated by short- and medium-range order.

Abstract

In this work, we revisit the fragile-to-strong transition (FTS) in the simulated BKS silica from the perspective of microscopic dynamics in an effort to elucidate the dynamical behaviors of fragile and strong glass-forming liquids. Softness, which is a machine-learned feature from local atomic structures, is used to predict the microscopic activation energetics and long-term dynamics. The FTS is found to originate from a change in the temperature dependence of the microscopic activation energetics. Furthermore, results suggest there are two diffusion channels with different energy barriers in BKS silica. The fast dynamics at high temperatures is dominated by the channel with small energy barriers ($<\sim$1 eV), which is controlled by the short-range order. The rapid closing of this diffusion channel when lowering temperature leads to the fragile behavior. On the other hand, the slow dynamics at low temperatures is dominated by the channel with large energy barriers controlled by the medium-range order. This slow diffusion channel changes only subtly with temperature, leading to the strong behavior. The distributions of barriers in the two channels show different temperature dependences, causing a crossover at $\sim$3100 K. This transition temperature in microscopic dynamics is consistent with the inflection point in the configurational entropy, suggesting there is a fundamental correlation between microscopic dynamics and thermodynamics.