The mixed alkali effect in ionically conducting glasses revisited: a study by molecular dynamics simulation

Area of Science:

• Materials Science
• Solid-State Chemistry
• Computational Physics

Background:

• The Mixed Alkali Effect (MAE) describes a non-linear decrease in ion transport coefficients when multiple mobile ion types are mixed in ionic conductors.
• This phenomenon is a long-standing, unsolved problem in the field of ionically conducting materials.

Purpose of the Study:

• To investigate the complex ion dynamics underlying the MAE in mixed alkali silicate glasses using molecular dynamics simulations.
• To identify the fundamental mechanisms responsible for the MAE, moving beyond traditional explanations.

Main Methods:

• Molecular dynamics (MD) simulations were employed to model ion transport in mixed alkali lithium-potassium silicate glasses.
• Analysis of van Hove functions was used to examine ion jump dynamics and site-to-site transitions.

Main Results:

• Distinct features in van Hove functions indicated suppressed ion jumps between sites in mixed alkali systems.
• Dynamical heterogeneity and "cooperativity blockage" due to ion-ion interactions were identified as fundamental drivers of the MAE.
• Suppressed long-range ion motion and increased correlated backward motions were linked to decreased diffusivity, especially at low concentrations of foreign alkali ions.

Conclusions:

• The MAE is fundamentally driven by ion-ion interactions and correlated dynamics, leading to dynamical heterogeneity.
• Traditional factors like network formers and unoccupied sites are not essential for explaining the MAE.
• The observed ion dynamics and MAE are consistent across various ionic conductors, including non-glassy systems, supporting the universality of these findings.