A New Superionic Plastic Polymorph of the Na+ Conductor Na3PS4

Na3PS4 is one of the most promising Na+ conductors, relevant for applications that can leverage its high ionic conductivity, such as solid-state batteries. Currently, two crystalline phases of the material have been identified, and it has been thought to melt above 500 degrees C. In contrast, based on diffraction, ab initio simulations, impedance spectroscopy, and thermal analysis, we show that Na3PS4 remains solid above this temperature and transforms to a third polymorph, gamma, exhibiting fast-ion conduction and an orthorhombic crystal structure. We show that the fast Na+-conduction is associated with rotational motion of the thiophosphate polyanions pointing to a plastic crystal. These findings are of major importance for the development and understanding of new polyanion-based solid electrolytes.