The cationic energy landscape in alkali silicate glasses: properties and relevance

TL;DR

This study uses molecular dynamics to analyze cationic site energies in alkali silicate glasses, revealing that static energy landscapes and Coulomb interactions are crucial but insufficient alone to describe ion transport, suggesting the need for new models.

Contribution

The paper explicitly determines cationic site energies from simulations and demonstrates the limitations of static hopping models in capturing ion dynamics.

Findings

Coulomb interactions are essential for accurate energy landscapes.

Static energy landscapes do not fully explain ion dynamics.

A single vacancy model shows the inadequacy of static energies.

Abstract

Individual cationic site--energies are explicitly determined from molecular dynamics simulations of alkali silicate glasses, and the properties and relevance of this local energetics to ion transport are studied. The absence of relaxations on the timescale of ion transport proves the validity of a static description of the energy landscape, as it is generally used in hopping models. The Coulomb interaction among the cations turns out essential to obtain an average energy landscape in agreement with typical simplified hopping models. Strong correlations exist both between neighboring sites and between different energetic contributions at one site, and they shape essential characteristics of the energy landscape. A model energy landscape with a single vacancy is used to demonstrate why average site--energies, including the full Coulomb interaction, are still insufficient to describe the site population of ions, or their dynamics. This model explains how the relationship between energetics and ion dynamics is weakened, and thus establishes conclusively that a hopping picture with static energies fails to capture all the relevant information. It is therefore suggested that alternative simplified models of ion conduction are needed.