The polymorphs of the Na+ion conductor Na3PS4viewed from the perspective of a group-subgroup scheme

Abstract

The Na+ solid state electrolyte Na3PS4 is currently being investigated due to its high ionic conductivity and its synthesis-dependent crystal structure. Na3PS4 adopts a tetragonal low-temperature modification with space group P 4̅21c that transforms to a cubic high-temperature modification with space group I 4̅3m (Tl3VS4 type). These two modifications are related by a group-subgroup scheme. The symmetry reduction proceeds via a translationengleiche transition from I 4̅3m to I 4̅2m and subsequently via a klassengleiche transition to P 4̅21c. The tetragonal phase with space group I 4̅2m corresponds to the K2HgSnSe4 type. The group-subgroup scheme of this tetragonal branch of the Bärnighausen tree is discussed along with the crystal chemical consequences and results of diffraction experiments. The structure of K3SbSe4 (space group R 3c) belongs to the rhombohedral branch of the aristotype Tl3VS4.