A New Phase of the Na+ Ion Conductor Na3PS4

Solid electrolytes are crucial for next‑generation solid‑state batteries and Na3PS4 is one of the most promising Na+ conductors for such applications. At present, two phases of Na3PS4 have been identified and it had been thought to melt above 500 °C. In contrast, we show that it remains solid above this temperature and transforms into a third polymorph, γ, exhibiting superionic behavior. We propose an orthorhombic crystal structure for γ‑Na3PS4 based on scattering density analysis of diffraction data and density functional theory calculations. We show that the Na+ superionic behavior is associated with rotational motion of the thiophosphate polyanions pointing to a rotor phase, based on ab initio molecular dynamics simulations and supported by high‑temperature synchrotron and neutron diffraction, thermal analysis and impedance spectroscopy. These findings are of importance for the development of new polyanion‑based solid electrolytes.