Superionic Conduction in the Plastic Crystal Polymorph of Na₄P₂S₆

Sodium thiophosphates are promising materials for large-scale energy storage applications benefiting from high ionic conductivities and the geopolitical abundance of the elements. A representative of this class is Na₄P₂S₆, which currently shows two known polymorphs-α and β. This work describes a third polymorph of Na₄P₂S₆, γ, that forms above 580 °C, exhibits fast-ion conduction with low activation energy, and is mechanically soft. Based on high-temperature diffraction, pair distribution function analysis, thermal analysis, impedance spectroscopy, and ab initio molecular dynamics calculations, the γ-Na₄P₂S₆ phase is identified to be a plastic crystal characterized by dynamic orientational disorder of the P₂S₆ ^4- anions translationally fixed on a body-centered cubic lattice. The prospect of stabilizing plastic crystals at operating temperatures of solid-state batteries, with benefits from their high ionic conductivities and mechanical properties, could have a strong impact in the field of solid-state battery research.