Anharmonic phonons, superionic diffusion, and ultralow thermal conductivity in argyrodite Cu₇PSe₆

We present a combined experimental and theoretical investigation of atomic dynamics in the superionic compound Cu₇PSe₆, rationalizing the atomistic diffusion mechanism and the impact of host lattice dynamics. Inelastic neutron scattering (INS) and quasi-elastic neutron scattering (QENS) were performed as a function of temperature, and were complemented with ab-initio molecular dynamics extended to long time scales with the use of machine-learned potentials (MLMD). INS data reveal characteristic changes at the onset of superionic behavior, providing insights into the role of the host lattice dynamics, while QENS probes the superionic Cu diffusion via the jump length, residence time and diffusion constant. The MLMD simulations reveal that the long-range Cu diffusion is limited by an inter-cluster hopping step, which is strongly coupled to the host phonon dynamics. Further, MLMD simulations enable us to capture the ultralow lattice thermal conductivity within the Green-Kubo framework. These results supersede the traditional quasiharmonic phonon picture to capture the strong anharmonicity in this superionic system.