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) Å(3) (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@Li4Ge12](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.