Diffusion and Lattice Dynamics in Na-based Superionic Conductors

The demand for next-generation all-solid-state batteries with higher energy density and improved safety motivates extensive efforts to develop superionic conductors (SICs) for solid-state electrolytes. Further, Na-based batteries could be attractive by virtue of the abundance of sodium. While several Na-based SICs have been discovered, the key atomistic processes enabling the fast Na ion diffusion remain poorly understood. Combining neutron scattering and advanced materials simulation, we performed detailed investigations of the interplay between fast Na diffusion and host lattice dynamics in Na₃SbS₄ and Na₃SbSe₄. These compounds crystallize in a tetragonal phase at low temperature and upon warming transform into a superionic cubic phase, with ionic conductivity of order of ~10^-3 S cm^-1. Our inelastic neutron scattering measurements reveal a strong spectral weight transfer from low-energy phonons to diffusive dynamics in the superionic phase, indicating that low-energy anharmonic phonons can be seen as precursors of ionic hopping. Our phonon dispersion simulations support this view, and identify strongly anharmonic transverse acoustic modes near the H point, involving coupled Na hopping attempts and SbX₄ tetrahedra dynamics. These results suggest that soft anharmonic phonons could play an important role in enabling fast Na diffusion in both Na₃SbS₄ and Na₃SbSe_4.