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\nhigh ionic conductivity, such as solid-state batteries. Currently,\ntwo crystalline phases of the material have been identified, and it\nhas been thought to melt above 500 °C. In contrast, based on diffraction, ab initio simulations,\nimpedance spectroscopy, and thermal analysis, we show that Na3PS4 remains solid above this temperature and transforms\nto a third polymorph, γ, exhibiting fast-ion conduction and\nan orthorhombic crystal structure. We show that the fast Na+-conduction is associated with rotational motion of the thiophosphate\npolyanions pointing to a plastic crystal. These findings are of major\nimportance for the development and understanding of new polyanion-based\nsolid electrolytes.