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

The Na+solid state electrolyte Na3PS4is currently being investigated due to its high ionic conductivity and its synthesis-dependent crystal structure. Na3PS4adopts a tetragonal low-temperature modification with space groupP 4̅21cthat transforms to a cubic high-temperature modification with space groupI 4̅3m(Tl3VS4type). These two modifications are related by a group-subgroup scheme. The symmetry reduction proceeds via atranslationengleichetransition fromI 4̅3mtoI 4̅2mand subsequently via aklassengleichetransition toP 4̅21c. The tetragonal phase with space groupI 4̅2mcorresponds to the K2HgSnSe4type. 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 groupR 3c) belongs to the rhombohedral branch of the aristotype Tl3VS4.