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 < 600 degrees C through combined experimental-computational analysis. Although Bragg diffraction experiments indicate a second-order phase transition from the tetragonal ground state (a, P (4) over bar2(1)c) to the cubic polymorph (beta, I (4) over bar 3m) above similar to 250 degrees C, pair distribution function analysis in real space and Raman spectroscopy indicate remnants of a tetragonal character in the range 250 < T < 500 degrees C, which we attribute to dynamic local tetragonal distortions. The first-order phase transition to the mesophasic high-temperature polymorph (gamma, 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.

Automated summary

This study investigates the structure and dynamics evolution of Na3PS4 over a wide temperature range through combined experimental-computational analysis, revealing the polymorphic transitions and dynamic behavior associated with different phases at varying temperatures. These findings provide insights into the rich polymorphism of Na3PS4 and its relevance for high-performance materials derived from its structural archetype.