Synthesis, Structure and Physical Properties of the Most Sodium‐Rich Phosphidogermanate Na8GeP4

Sodium ion conductor materials hold promise to be an affordable substitute for ever growing lithium demand. To achieve high ionic conductivity in all‐solid state ion conductor materials, fundamental understanding of the atomic structure of the materials as well as the structure‐property relationship are basic prerequisites. It has been recently shown that phosphide‐based materials such as the superionic conductor Li9AlP4 or the most lithium‐rich Li14SnP6 show great ionic conductivities what makes them promising candidates for solid electrolyte materials. Here, we present the investigation of the most sodium rich phase Na8GeP4 that reveals a similar structure if compared to the lithiated phases. Na8GeP4 is synthesized via a ball‐milling procedure and a subsequent annealing process. Crystallographic investigation by Rietveld method shows that Na8GeP4 crystallizes in the cubic space group Fdm (no. 227) with a cell parameter of a = 13,4230 Å at 298K. While it shares many structural motifs of Li8GeP4, it is not isotypic but crystallizes in the Na8SnSb4 structure type. Impedance spectroscopy as well as density functional calculations reveal that Na8GeP4 is a good electronic conductor with a band gap of 1.9 eV.