Zintl Phases K_4–xNa_xSi₄(1 ≤ x ≤ 2.2) and K₇NaSi₈: Synthesis, Crystal Structures, and Solid-State NMR Spectroscopic Investigations

The Zintl phases K_4–xNa_xSi₄(1 ≤ x ≤ 2.2) and K₇NaSi₈are the first representatives of the K–Na–Si system, and both contain tetrahedral [Si₄]^4–clusters and a charge-balancing number of K⁺and Na⁺cations. All phases of K_4–xNa_xSi₄(1 ≤ x ≤ 2.2) crystallize in a new structure type with space group P2₁/n, as determined by single-crystal X-ray diffraction analysis of the parent phase K₃NaSi₄. Rietveld refinement of the X-ray diffraction data showed that the solid solutions follow Vegard's rule. K₇NaSi₈can only be synthesized by diffusion-controlled reaction of binary precursors and is isostructural with known A₇A′T₈(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 K₇NaSi₈, which is additionally compared with the related phases A₇NaSi₈(A = Rb, Cs).