Switching the Structure Type upon Ag Substitution: Synthesis and Crystal as well as Electronic Structures of Li12AgGe4

Li-rich compounds of metals and semimetals are interesting candidates for anode materials for rechargeable batteries. The investigation of the Li-rich part of the Li–Ag–Ge phase diagram led to the discovery of the new compound Li12AgGe4, which represents the Li-richest phase in the ternary phase system. The phase-pure compound is synthesized by high-temperature reaction of Li with stoichiometric amounts of premelted reguli of Ag and Ge. The structure was determined by single-crystal X-ray diffraction. Li12AgGe4 crystallizes in the Li13Si4 structure type in the space group Pbam (no. 55) with lattice parameters of a = 8.0420(2) Å, b = 15.1061(4) Å, and c = 4.4867(1) Å and exhibits the unique Zintl anion [AgGe2]7–iso(valence) electronic to the CO2 moleculeand Ge2 dumbbells. Li12AgGe4 adopts the atom packing of the lighter homologue Li13Si4 and not that of Li13Ge4 by the selective substitution of one out of seven Li positions by Ag. The calculation of the electronic structure indicates metallic property and the presence of strong covalent bonds between Ag and Ge in the linear triatomic Ge–Ag–Ge unit as well as π character between the Ge atoms of the dumbbells. The Ag–Ge bond order of the linear AgGe2 unit reaches its maximum at EF of Li12AgGe4 with full occupancy of all atomic positions (in contrast to the related Li12Ag1–xSi4), indicating that the formation of covalent Ag–Ge bonds is the driving force for the formation of the structure type.