The structural relaxation of molten sodium disilicate

TL;DR

This study uses molecular dynamics simulations to analyze the relaxation dynamics of molten sodium disilicate, revealing anomalous sodium atom behavior and confirming mode-coupling theory predictions for relaxation times and correlators.

Contribution

It provides detailed insights into the relaxation mechanisms of Na2O-2(SiO2) in its molten state, highlighting the role of channel-like structures and validating mode-coupling theory.

Findings

Na atoms relax 100 times faster than Si and O at low temperatures

Coherent functions relax on the same time scale at wave-vector ~1Å^-1

Diffusion constants follow power-law behavior predicted by mode-coupling theory

Abstract

We use molecular dynamics computer simulations to study the relaxation dynamics of Na2O-2(SiO2) in its molten, highly viscous state. We find that at low temperatures the incoherent intermediate scattering function for Na relaxes about 100 times faster than the one of the Si and O atoms. In contrast to this all coherent functions relax on the same time scale if the wave-vector is around 1AA^-1. This anomalous relaxation dynamics is traced back to the channel-like structure for the Na atoms that have been found for this system. We find that the relaxation dynamics for Si and O as well as the time dependence of the coherent functions for Na can be rationalized well by means of mode-coupling theory. In particular we show that the diffusion constants as well as the alpha-relaxation times follow the power-law predicted by the theory and that in the beta-relaxation regime the correlators obey the factorization property with a master curve that is described well by a von Schweidler-law. The value of the von Schweidler exponent $b$ is compatible with the one found for the mentioned power-law of the relaxation times/diffusion constants. Finally we study the wave-vector dependence of f_s(q) and f(q), the coherent and incoherent non-ergodicity parameters. For the Si and O atoms these functions look qualitatively similar to the ones found in simple liquids or pure silica, in that the coherent function oscillates (in phase with the static structure factor) around the incoherent one and in that the latter is approximated well by a Gaussian function. In contrast to this, f(q) for Na-Na is always smaller than f_s(q) for Na and the latter can be approximated by a Gaussian only for relatively large q.