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

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 < 600 °C through combined experimental-computational analysis. Although Bragg diffraction experiments indicate a second-order phase transition from the tetragonal ground state (α, P4̅2₁ c) 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 < 500 °C, which we attribute to dynamic local tetragonal distortions. The first-order phase transition to the mesophasic high-temperature polymorph (γ, 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.