Breakdown of Soft Anharmonic Phonons Heralds Fast Ionic Diffusion in Lithium Argyrodite

A fundamental understanding of the atomic structure and dynamics enabling fast ionic transport in solids is essential for the development of next-generation solid-state electrolytes (SSE). Focusing on the promising SSE candidate Li₆PS₅Cl with argyrodite structure, we resolve the coupling between fast diffusion of Li⁺ and vibrational dynamics of the host framework through extensive inelastic and quasielastic neutron scattering measurements, combined with machine-learned molecular dynamics (MLMD) simulations based on first-principles data. Our results establish that host lattice vibrations enable an order-of-magnitude increase in Li⁺ diffusivity at ambient temperature. Our experiments and simulations both show a clear overlap and interplay of hopping dynamics and vibrational frequencies in the terahertz regime, with a continuous spectral evolution from harmonic phonons to strongly anharmonic overdamped vibrations, and fast Li⁺ diffusion. We identify the key degrees-of-freedom enabling fast Li diffusion as low-frequency dynamics of PS₄^3-polyanions, which are distinct from the commonly assumed "paddle-wheel" scenario. Bringing together neutron measurements and large-scale MLMD simulations, our results build a "beyond phonons" picture of complex atomic dynamics in SSEs in terms of overdamped spectral functions. These results offer microscopic insights into the mechanism of fast Li⁺ diffusion in lithium argyrodites and provide guidance for the design of future SSE materials.