Coupling of Ionic Hopping and Anharmonic Phonon Dynamics in Superionic Conductors for Solid State Electrolytes

<br/>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 Na3MX4 and their coupling to fast Na diffusion, using a combination of neutron sca6ering, ab-initio molecular dynamics (AIMD), and accelerated molecular dynamics based on machine-learned potentials. We identify anharmonic soft-modes of the high-temperature phase that play an important role as precursors to mobile Na hopping. We also show how these strongly anharmonic phonon modes enable Na-ions to hop along the minimum energy pathways. Further, the quasi-elastic neutron sca6ering measurements, supplemented with large-scale molecular dynamics simulations, probe the Na diffusion constant and the diffusion characteristics. These results offer detailed microscopic insights into the dynamic mechanism of fast Na diffusion and provide an avenue to search for further Na 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.