Investigation of Low-Energy Lattice Dynamics and Their Role in Superionic Na Diffusion and Ultralow Thermal Conductivity of Na₃PSe₄ as a Solid-State Electrolyte

The atomic dynamics of Na₃PSe₄ were investigated using a combination of neutron scattering experiments and ab initio and machine-learned molecular dynamics simulations to probe the interplay of fast ionic diffusion with atomic vibrations (phonons) of the host lattice. Our results reveal the existence of low-energy vibrational modes, simultaneously involving motions of Na⁺ ions and framework polyanion subunits, and show that these modes become strongly overdamped in the superionic regime as they couple with the Na⁺ hopping process. In particular, the Na⁺ migration energy landscape is strongly impacted by low-energy phonons derived from a soft acoustic branch of the host lattice, which modulates the diameter of the Na⁺ diffusion channel at the bottleneck. We find that an additional factor for the enhanced Na⁺ conductivity in Na₃PSe₄ is the presence of Na-vacancies, which also affect the low-frequency dynamics and thermal vibration amplitudes, pointing to an interplay between Na⁺ vacancies and host dynamics, jointly enhancing ionic diffusivity. Finally, we investigate the origin of ultralow thermal conductivities in Na₃PSe₄ and Na₃PS₄ using Green-Kubo simulations and find that low-energy acoustic phonon modes of the overall crystal framework provide a dominant contribution to the thermal conductivity.