Here, we elucidate the role of collective vibrational dynamics in governing the ionic transport in solids. It is demonstrated that ionic transport is mediated through a synergistic interplay between unstable and stable vibrational modes. Unstable vibrational modes directly initiate the hopping of ions by displacing ions from their equilibrium sites. Stable vibrational modes, in parallel, facilitate ion diffusion indirectly by amplifying the separation of local ionic pairs through coordinated collective rearrangements. The interaction between unstable and stable vibrational modes collectively results in a high ionic diffusivity in solids. The “vibration-ion” interaction is further designed via defect engineering. Our results show that the population of unstable vibrational modes is increased by tailoring the vibrational spectrum using intrinsic ionic vacancies. This concurrently strengthens the direct ionic translation and the coupling between unstable and stable vibrational modes. As a result, the ionic diffusivity of the example system ${\mathrm{Ag}}_2\mathrm{Te}$ increases from $0.84\times {10}^{-5}$ to $1.54\times {10}^{-5}{\mathrm{cm}}^2/\mathrm{s}$ at 500 K when $10\%{\mathrm{Te}}^{2-}$ vacancies are introduced. Our Letter establishes a physical picture to bridge the microscopic vibrational dynamics to macroscopic diffusion properties.

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• Diffusion
• Ionic transport
• Molecular dynamics