Phonon-Activated Collective Hopping in a Superionic Phase

Superionic conductors are fascinating systems representing a state of matter intermediate between solids and liquids, yet their underlying physics remain poorly understood. Correlations in jump dynamics have been theoretically proposed as a key mechanism underlying superionic diffusion, but a direct experimental demonstration remains elusive. Here, we report the direct observation of collective cation hopping in the argyrodite compound Cu 7 PS 6 , using single-crystal neutron scattering to access the full four-dimensional space-time correlations of ionic motion. By leveraging the rarely used coherent scattering channel in quasielastic neutron scattering, together with diffuse scattering, we reveal pronounced short-range dynamical correlations between diffusing Cu ions, providing direct experimental evidence of collective hopping in a superionic phase. Machine-learned molecular dynamics simulations quantitatively reproduce all our measurements and pinpoint the dominant collective hopping mechanism. Furthermore, we establish through both experiments and simulations that the collective diffusive hops originate from the breakdown of low-energy phonons dominated by the Cu motions in the parent ordered phase, demonstrating a key link between phonon precursors and the activation of facile collective hops. This work presents the first direct experimental evidence of collective hopping in superionic argyrodites and identifies wave-vector-resolved soft phonons as its physical origin, advancing the microscopic understanding of ionic transport in superionic solids.