Synthesis, Structure and Physical Properties of the 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 lithium-rich Li₁₄SnP₆ show great ionic conductivities what makes them promising candidates for solid electrolyte materials. Here, we present the investigation of the 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 Fd (Figure presented.) m (no. 227) with a cell parameter of a=13.4230 Å at 298 K. 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 theory calculations reveal that Na₈GeP₄ is a good electronic conductor with a band gap of 1.9 eV.