The Polymorphs 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. In this contribution, we present a detailed investigation of the evolution in structure and dynamics of Na3PS4 under the effect of temperature in the range 30 < T < 600 °C through combined experimental-computational analysis. Although x ray Bragg diffraction experiments indicate a second order phase transition from the tetragonal ground state (α, P-421c) to the cubic polymorph (β, I-43m), pair distribution function analysis in real space and Raman spectroscopy indicate remnants of 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 diffusive dynamics for sodium-cations (translative) and thiophosphate-polyanions (rotational) evident by inelastic neutron- and Raman- spectroscopies, as well as pair-distribution function and molecular dynamics. These results shed light on the rich polymorphism in Na3PS4 and are relevant for a host of high performance materials deriving from the Na3PS4 structural archetype.