Insights into the Rich Polymorphism of the Na+ Ion Conductor Na3PS4 from the Perspective of Variable-Temperature Diffraction and Spectroscopy

Solid electrolytes are crucial for next-generation solid-state batteries, and Na3PS4 is one of the most promising Na+ conductors for such applications, despite outstanding questions regarding its structural polymorphs. In this contribution, we present a detailed investigation of the evolution in structure and dynamics of Na3PS4 over a wide temperature range 30 T P4̅21c) to the cubic polymorph (β, I4̅3m) above ∼250 °C, pair distribution function analysis in real space and Raman spectroscopy indicate remnants of a tetragonal character in the range 250 T Fddd) is associated with a sharp volume increase and the onset of liquid-like dynamics for sodium-cations (translational) and thiophosphate-polyanions (rotational) evident by inelastic neutron and Raman spectroscopies, as well as pair-distribution function and molecular dynamics analyses. These results shed light on the rich polymorphism of Na3PS4 and are relevant for a range host of high-performance materials deriving from the Na3PS4 structural archetype.