Lithium-Stuffed Diamond Polytype Zn–Tt Structures (Tt = Sn, Ge): The Two Lithium–Zinc–Tetrelides Li3Zn2Sn4 and Li2ZnGe3

In view of the search for alternative structures for Li ion battery materials and electron-poor framework semiconductors for thermoelectric applications, the systems Li–Zn–Tt with Tt = Ge or Sn were investigated. Li3Zn2Sn4 and Li2ZnGe3 were obtained by high-temperature syntheses from the elements. The crystal structures of both phases were determined with single-crystal X-ray diffraction methods and the electronic structure of Li3Zn2Sn4 was analyzed by means of DFT calculations (TB-LMTO-ASA). Both phases show diamond polytype analogous Zn–Tt networks with tetrahedrally four-coordinated Zn and Tt atoms. The new phase Li3Zn2Sn4 crystallizes in space group P63/mmc (No. 194) with lattice parameters a = 4.528(1) Å and c = 22.119(2) Å. Zn and Sn atoms are fully ordered on three sites that constitute a 6H diamond polytype like network. Li2ZnGe3 is also described in space group P63/mmc (No. 194) with lattice parameters a = 4.167(1) Å and c = 6.754(1) Å. The Zn–Ge substructure is a hexagonal diamond (2H polytype) like network. The existence of such a Ge-rich Li–Zn–Ge phase has already been reported, but a full structure determination has not yet been published. No indication for an ordering of Zn and Ge atoms on different sites could be deduced from the X-ray diffraction data. Band structure calculations for Li3Zn2Sn4 indicate that the phase is metallic, with the Fermi level at the flank of a pseudogap in the density of states curve. The topological analysis of the electron localization function (ELF) shows covalent Sn–Sn bonding and lone pair like valence basins for the Sn atoms. Concerning the appearance of the lone pair like ELF basins, a strong influence of the basis set for Li that is employed in the calculations is found.