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 <i>Pbam</i> (no. 55) with lattice parameters of <i>a</i> = 8.0420(2) Å, <i>b</i> = 15.1061(4) Å, and <i>c</i> = 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 <i>E</i>_F of Li₁₂AgGe₄ with full occupancy of all atomic positions (in contrast to the related Li₁₂Ag_1–<i>x</i>Si₄), indicating that the formation of covalent Ag–Ge bonds is the driving force for the formation of the structure type