Lithium-stuffed diamond polytype Zn-Tt structures (Tt = Sn, Ge): The two lithium-zinc-tetrelides Li_3Zn_2Sn_4 and Li _2ZnGe_3

摘要

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 _3Zn_2Sn_4 and Li_2ZnGe_3 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 Li_3Zn_2Sn _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 Li_3Zn _2Sn_4 crystallizes in space group P6_3/mmc (No. 194) with lattice parameters a = 4.528(1) ? and c = 22.119(2) ?. 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) ? and c = 6.754(1) ?. 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 Li_3Zn_2Sn_4 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.