Superionic materials represent a regime intermediate between the crystalline and liquid states of matter. Despite the considerable interest in potential applications for solid-state batteries or thermoelectric devices, it remains unclear whether the fast ionic diffusion observed in superionic materials reflects liquid-like dynamics or whether the hops of mobile ions are inherently coupled to more conventional lattice phonons.

Here, a crossover was revealed from crystalline vibrations to relaxational dynamics of ionic diffusion in the superionic compound Li₆PS₅Cl, a candidate solid-state electrolyte. By combining inelastic and quasi-elastic neutron-scattering measurements with first-principles-based machine-learned molecular dynamics simulations, it was found that the vibrational density of states in the superionic state strongly deviates from the quadratic behavior expected from the Debye law of lattice dynamics.

The superionic dynamics emerges from overdamped phonon quasiparticles to give rise to a linear density of states characteristic of instantaneous normal modes in the liquid state. Further, it was shown that the coupling of lattice phonons with a dynamic breathing of the Li⁺ diffusion bottleneck enables an order-of magnitude increase in diffusivity. Thus, these results shed insights into superionics for future energy storage and conversion technologies.