From lithium to sodium – structural investigation of sodium phosphidogermanates Manuel Botta , Sabine Zeitz, Prof. Dr. Thomas F. Fässler Department of Chemistry, Chair of Inorganic Chemistry with Focus on New Materials, Technische Universität München, Lichtenbergstraße 4, D-85747 Garching, Germany Sodium ion conductor materials hold promise as a component in all solid-state batteries (ASSBs) 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. Consequently, the systematic investigation of structural parameters in such material families is required. It was recently shown that phosphide based materials such as the superionic conductor α- and β-Li 8 GeP 4 [1] or the most lithium-rich Li 14 GeP 6 [2] show great ionic conductivities what makes them promising candidates for solid electrolyte materials. Hence we started our investigation on new sodium solid electrolyte materials in this phosphide based system. Here we report on the investigation of the system Na-Ge-P and the synthesis and structure determination of two new phases. The so far sodium richest phase Na 8 GeP 4 reveals a very closely related crystallographic structure to the lithiated phases Li 8 GeP 4 . [1] Structure determination by powder X-Ray diffraction data utilizing the Rietveld method revealed isolated tetrahedral units separated by Na + ions in Na 8 GeP 4 . Impedance spectroscopy, DC polarization measurements as well as density functional theory calculations reveal that Na 8 GeP 4 has a very high electronic conductivity and a band gap of 1.7 eV. Crystallographic investigation of Na 3 Ge 2 P 3 was performed by the means of single crystal X-ray structure determination. In contrast to isolated tetrahedral units, it forms an condensed two‑dimensional polyanionic network with interconnected dimeric tetrahedral units that contain covalent homoatomic Ge-Ge bonds besides Ge-P bonds as reported for Na 2 Ge 3 P 3 and Na 5 Ge 7 P 5 . [3] This emphasizes the great structural variety of such compounds. Computational investigation of the electronic structure reveals that Na 3 Ge 2 P 3 is an indirect band gap semiconductor with a band gap of 2.9 eV. References 1. H. Eickhoff, S. Strangmüller, W. Klein, H. Kirchhain, C. Dietrich, W. G. Zeier, L. van Wüllen, T. F. Fässler, Chemistry of Materials 2018 , 30 , 6440-6448. 2. S. Strangmüller, H. Eickhoff, G. Raudaschl-Sieber, H. Kirchhain, C. Sedlmeier, L. van Wüllen, H. A. Gasteiger, T. F. Fässler, Chemistry of Materials 2020 , 32 , 6925-6934. 3. H. Eickhoff, V. Hlukhyy, T. F. Fässler, Zeitschrift für anorganische und allgemeine Chemie 2020 , 646 , 1834-1838.
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