We report a new sodium superionic conductor, Na11Sn2PS12, that crystallizes in an unprecedented three-dimensional structure type and exhibits an ionic conductivity of 1.4 mS cm−1, with a very low activation energy barrier for Na-ion mobility of 0.25 eV. A combination of structural elucidation via single crystal X-ray diffraction and ab initio molecular dynamics simulations show that Na+-ion conduction pathways flow through equi-energetic sodium–sulfur octahedra interconnected by partial vacancy cross-over sites in all crystallographic dimensions, providing an understanding of the underlying isotropic 3D fast-ion conduction in this material.
