Phonons and Lithium diffusion in LiAlO$_2$

TL;DR

This study combines inelastic neutron scattering and ab-initio molecular dynamics to analyze phonons and lithium diffusion in LiAlO$_2$, revealing super-ionic behavior at high temperatures and the influence of structural disorder on diffusion pathways.

Contribution

It provides new insights into lithium diffusion mechanisms and phonon behavior in LiAlO$_2$, especially highlighting the effects of vacancies and amorphous phases on diffusion.

Findings

Phonon modes related to Li vibrations broaden with temperature.

Super-ionic lithium diffusion occurs at high temperatures in all structures.

Amorphous and vacancy structures facilitate easier lithium diffusion.

Abstract

We report on investigations of phonons and lithium diffusion in LiAlO$_2$ based on inelastic neutron scattering (INS) measurements of the phonon density of states (DOS) in {\gamma}-LiAlO$_2$ from 473 K to 1073 K, complemented with ab-initio molecular dynamics (AIMD) simulations. We find that phonon modes related to Li vibrations broaden on warming as reflected in the measured phonon DOS and reproduced in simulations. Further, the AIMD simulations probe the nature of lithium diffusion in the perfect crystalline phase ({\gamma}-LiAlO$_2$), as well as in a structure with lithium vacancies and a related amorphous phase. Almost liquid-like super-ionic diffusion is observed in AIMD simulations of the three structures at high temperatures; with predicted onset temperatures of 1800 K, 1200 K, and 600 K in the perfect structure, vacancy structure and the amorphous phase, respectively. In the ideal structure, the Li atoms show correlated jumps; while simple and correlated jumps are both seen in the vacancy structure, and a mix of jumps and continuous diffusion occur in the amorphous structure. Further, we find that the Li-diffusion is favored in all cases by a large librational amplitude of the neighbouring AlO4 tetrahedra, and that the amorphous structure opens additional diffusion pathways due to a broad distribution of AlO4 tetrahedra orientations.