Li₁₈Na₂Ge₁₇ - A compound demonstrating cation effects on cluster shapes and crystal packing in ternary Zintl phases

The novel ternary Zintl phase Li₁₈Na₂Ge₁₇ was synthesized from a stoichiometric melt and characterized crystallographically. It crystallizes in the trigonal space group P31m (No. 157) with a = 17.0905(4) Å, c = 8.0783(2) Å, and V = 2043.43(8) ų (final R indices R1 = 0.0212 and wR2 = 0.0420 for all data). The structure contains three different Zintl anions in a 1:1:1 ratio: isolated anions Ge^4-, tetrahedra [Ge₄]^4-, and truncated, Li-centered tetrahedra [Li@Ge₁₂]^11-, whose hexagonal faces are capped by four Li cations, resulting in the Friauf polyhedra [Li@Li₄Ge₁₂]^7-. According to the Zintl-Klemm concept, Li₁₈Na₂Ge₁₇ is an electronically balanced Zintl phase, as experimentally verified by its diamagnetism. The compound is structurally related to Li₇RbGe₈, which also contains [Ge₄]^4- and [Li@Li₄Ge ₁₂]^7- in its anionic substructure. However, exchanging the heavier alkali metal cation Rb for Na in the mixed-cation germanides leads to drastic changes in stoichiometry and crystal packing, demonstrating the great effects that cations exert on such Zintl phases through optimized cluster sheathing and space filling.