Anharmonic Phonons, Superionic Diffusion and Ultralow Thermal Conductivity in Complex Argyrodite Cu₇PSe₆

Complex argyrodite compounds are attracting intense interest for both solid-state electrolytes owing to their superionic behavior, and for thermoelectric applications owing to their ultralow thermal conducitivities. We present a combined experimental and theoretical investigation of atomic dynamics in the complex superionic argyrodite compound Cu₇PSe₆, rationalizing the atomistic diffusion mechanism and the influence of its soft, anharmonic 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. 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, and directly match the experimental QENS results. Further, MLMD simulations enable us to capture the ultralow lattice thermal conductivity and its various contributions beyond conventional propagative phonons. These results provide detailed insights into the hybrid atomic dynamics in superionic argyrodites, and supersede the traditional quasiharmonic phonon picture to capture the strong anharmonicity in this complex superionic system.