Theory for the mixed alkali effect in glasses

Area of Science:

• Materials Science
• Solid State Physics
• Physical Chemistry

Background:

• The mixed alkali effect (MAE) describes significant nonlinear changes in ionic transport properties when different mobile ions are mixed in a glassy material.
• Understanding MAE is crucial for designing glasses with tailored electrical and ionic conductivity for various applications.

Purpose of the Study:

• To develop a comprehensive theory for the mixed alkali effect based on thermally activated hopping transport.
• To incorporate statistical-mechanical and kinetic aspects of mobile ion mixtures into a theoretical framework.
• To explain the origins of MAE, even when ion species share similar site energy distributions.

Main Methods:

• Developed a theory for thermally activated hopping transport in disordered site energy landscapes.
• Incorporated joint probability density of site energy states and generalized Fermi distributions.
• Included cross terms in current response using nondiagonal Onsager coefficients.
• Validated the theory with kinetic Monte Carlo simulations.

Main Results:

• The theory demonstrates that MAE can occur even with identical site energy distributions for different ion species.
• Distinct site energies upon occupation by different ion types are sufficient to induce MAE.
• Mismatch energy for ion occupation and spatial correlations in site energies enhance the MAE.
• The model accurately predicts conductivity activation energies in mixed alkali phosphate glasses.

Conclusions:

• The developed theory provides a consistent explanation for the mixed alkali effect in glasses.
• The findings highlight the importance of ion-site energy interactions and spatial correlations in ionic transport.
• The theory offers a valuable tool for predicting and optimizing ionic transport properties in mixed-ion glasses.