Zintl Phases K 4– x Na x Si 4 (1 ≤ x ≤ 2.2) and K 7 NaSi 8 : Synthesis, Crystal Structures, and Solid‐State NMR Spectroscopic Investigations

The Zintl phases K 4– x Na x Si 4 (1 ≤ x ≤ 2.2) and K 7 NaSi 8 are the first representatives of the K–Na–Si system, and both contain tetrahedral [Si 4 ] 4– clusters and a charge‐balancing number of K + and Na + cations. All phases of K 4– x Na x Si 4 (1 ≤ x ≤ 2.2) crystallize in a new structure type with space group P 2 1 / n , as determined by single‐crystal X‐ray diffraction analysis of the parent phase K 3 NaSi 4 . Rietveld refinement of the X‐ray diffraction data showed that the solid solutions follow Vegard's rule. K 7 NaSi 8 can only be synthesized by diffusion‐controlled reaction of binary precursors and is isostructural with known A 7 A′T 8 ( A = Na–Cs; A′ = Li, Na; T = Si, Ge) phases. A combination of solid‐state NMR investigations and quantum mechanical calculations served to show the anisotropic chemical bonding behavior of all the atoms in K 7 NaSi 8 , which is additionally compared with the related phases A 7 NaSi 8 ( A = Rb, Cs).