Theory of defect-mediated ionic transport in Li, Na and K beta and beta prime prime aluminas

TL;DR

This study uses density functional theory to analyze defect structures in alkali metal beta and beta prime prime aluminas, revealing defect types and migration barriers that influence their ionic conductivity.

Contribution

It provides new insights into defect-mediated ion transport mechanisms in beta aluminas through detailed DFT calculations and migration energy analysis.

Findings

Charge transport dominated by interstitials in beta-aluminas.

Vacancies dominate in beta''-aluminas.

Predicted low activation energy of 20 meV for K beta'' alumina.

Abstract

Alkali metal $\beta$/$\beta^{\prime\prime}$ aluminas are among the fastest ionic conductors, yet little is understood about the role of defects in the ion transport mechanism. Here, we use density functional theory (DFT) to investigate the crystal structures of $\beta$ and $\beta^{\prime\prime}$ phases, and vacancy and interstitial defects in these materials. We find that charge transport is likely to be dominated by alkali metal interstitials in $\beta$-aluminas and by vacancies in $\beta^{\prime\prime}$ aluminas. Lower bounds for the activation energy for diffusion are found by determining the minimum energy paths for defect migration. The resulting migration barriers are lower than the experimental activation energies for conduction in Na $\beta$ and $\beta^{\prime\prime}$ aluminas, suggesting a latent potential for optimization. The lowest activation energy of about 20 meV is predicted for correlated vacancy migration in K $\beta^{\prime\prime}$ alumina.