Investigations of Coupled Dynamics of Mobile Ions and Anharmonic Phonons in Solid Electrolytes

The design of new solid-state electrolytes (SSEs) hinges on identifying and tuning relevant descriptors. While static structural descriptors have been correlated with fast diffusion, the host framework flexibility and vibrations and their complex dynamic coupling with mobile ions remain less studied. Phonons describe the atomic dynamics in crystalline materials and provide a basis to encode possible minimum energy pathways for ion migration but anharmonic effects can be large in SSEs. Identifying and controlling the pertinent phonon modes coupled most strongly with ionic conductivity, and assessing the role of anharmonicity, could therefore pave the way for discovering and designing new SSEs via phonon engineering. Here, we investigate phonons in a series of halide and sulfide fast Na+/Li+ ion conductors, using a combination of neutron scattering, ab-initio molecular dynamics (AIMD), and machine-learning molecular dynamics (MLMD). Using inelastic neutron scattering (INS) and MD simulations, we find that soft anharmonic phonon modes play an important role in facilitating ionic hops, reflecting the softness in the potential energy surface. Further, the quasi-elastic neutron scattering (QENS) measurements, supplemented with large-scale MD simulations, probe the diffusivity and the diffusion characteristics. These results offer detailed microscopic insights into the dynamic mechanism of fast ion diffusion and provide an avenue to search for further Na+/Li+ solid electrolytes. These results will also be contrasted with studies of coupled dynamics of anharmonic phonons and fast diffusion of mobile ions in other superionic systems.