Lithium-stuffed diamond polytype Zn-Tt structures (Tt = Sn, Ge): The two lithium-zinc-tetrelides Li₃Zn₂Sn₄ and Li ₂ZnGe₃

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. Li ₃Zn₂Sn₄ 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 Li₃Zn₂Sn ₄ 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 Li₃Zn ₂Sn₄ 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.