Enhancement of ion diffusion by targeted phonon excitation

Ion diffusion is important in a variety of applications, yet fundamental understanding of the interaction of phonons and the mobile species in solids is still missing. In this work, we introduce two formalisms that determine the individual contributions of phonons to the diffusion of ions through a solid, based on nudged elastic-band calculations and molecular dynamics simulations. The results for a model ion conductor of Ge-substituted Li₃PO₄ (Li_3.042Ge_0.042P_0.958O₄) reveal that more than 87% of the Li⁺ ion diffusion originates from less than 10% of the vibrational modes between 8 and 20 THz. By deliberately exciting a small, targeted subset of these contributing modes (<1%) to a higher temperature and still keeping the lattice at a low temperature, we observe an increase in diffusivity by several orders of magnitude. This new understanding identifies new avenues for increasing diffusivity by engineering the vibrations in a material without necessarily changing the compound chemistry.