Phonons and lithium diffusion in LiAlO2

We report on investigations of phonons and lithium diffusion in LiAlO2 based on inelastic neutron scattering measurements of the phonon density of states (DOS) in gamma-LiAlO2 from 473 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-LiAlO2), as well as in a structure with lithium vacancies and a related amorphous phase. In all three structures, almost liquidlike superionic diffusion is observed in AIMD simulations at high temperatures, with predicted onset temperatures of 1800, 1200, 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 occurs in the amorphous structure. Further, we find that Li diffusion is facilitated in all cases by a large librational amplitude of the neighboring AlO4 tetrahedra and that the amorphous structure opens additional diffusion pathways due to a broad distribution of AlO4 tetrahedra orientations.