Anharmonic lattice dynamics dominated ion diffusion in γ-Li3PO4

Understanding the influences of lattice dynamics on ionic transport holds the potential to shed light on advanced strategies for designing ionic conductors. However, their utilization remains infrequent due to a limited grasp of anharmonic lattice dynamics within ionic conductors. Herein, we focused on the anharmonic lattice dynamics of γ-Li3PO4, a prototype material for lithium superionic conductor, by combining x-ray diffraction, in situ Raman spectra and first principles calculation. We found that the diffusion process of Li-ion is primarily determined by two Raman modes, with one responsible for the activity of Li-ion and the other dominating the diffusion process. The two Raman modes all show strong anharmonicity and decay into three acoustical phonon modes even in low-temperature region. Our findings indicate that the diffusivity in ionic conductors can be increased by phonon engineering through driving the specific phonons without necessarily changing the compound chemistry.