Fast diffusion enabled by soft phonons in superionic crystals

Superionic conductors are fascinating materials, intermediate between the solid and liquid phases of matter. They are attracting considerable practical interest as their superior ionic diffusivity could enable applications as solid-state batteries and fuel cells. However, the key atomistic processes enabling fast ionic diffusion remain poorly understood. By combining extensive neutron scattering experiments with large-scale materials simulations, we obtain new insights into the atomistic mechanism enabling liquid-like diffusivities in crystals. We determine the key interplay between ionic hops and the lattice dynamics of the host crystal lattice. Through inelastic neutron scattering on single crystals, we establish that soft phonon modes at specific wave-vectors are strongly coupled to the superionic transition, acting as precursors of ionic hopping. Upon warming, these soft modes break down and give rise to a Q-dependent diffuse signal and coherent quasi-elastic response. The spectral weight transfer resulting from the breakdown of the soft modes is associated to the fast hopping dynamics. Our molecular dynamics simulations, based on a machine-learned potential, quantitatively reproduce the experimental observations, and further reveal the collective diffusion mechanism that preserves short-range order in the mobile ion subsystem. These results establish a direct connection between soft phonons and concerted diffusive dynamics, highlighting the role of anharmonic phonons in enabling fast ion diffusion in superionic conductors.