The synthesis of clathrate-II germanium □24Ge136 in an ionic liquid was recently discovered. The key step in this process, the controlled oxidation of reactive precursor phases by a protic acid, was successfully applied to silicides M4Si4 (M = Na, K). Through this method, clathrate-I compounds of composition M8- x Si46 were obtained as crystalline products. The synthetic method demonstrates a mild oxidative route to a large class of metastable intermetallic compounds.

The Zintl phase Ba 3 Si 4 has been synthesized from the elements at 1273 K as a single phase. No homogeneity range has been found. The compound decomposes peritectically at 1307(5) K to BaSi 2 and melt. The butterfly-shaped Si 4 6Ϫ Zintl anion in the crystal structure of Ba 3 Si 4 (Pearson symbol tP28, space group P4 2 /mnm, a ϭ 8.5233(3) Å , c ϭ 11.8322(6) Å ) shows only slightly different Si-Si bond lengths of d(SiϪSi) ϭ 2.4183(6) Å (1ϫ) and 2.4254(3) Å (4ϫ). The compound is diamagnetic with χ ഠ Ϫ50 ϫ 10 Ϫ6 cm 3 mol Ϫ1 . DC resistivity measurements show a high electrical resistivity (ρ(300 K) ഠ 1.2 ϫ 10 Ϫ3 Ω m) with positive temperature gradient dρ/dT. The temperature dependence of the isotropic signal shift and the spin-lattice relaxation times in 29 Si 1651 NMR spectroscopy confirms the metallic behavior. The experimental results are in accordance with the calculated electronic band structure, which indicates a metal with a low density of states at the Fermi level. The electron localization function (ELF) is used for analysis of chemical bonding. The reaction of solid Ba 3 Si 4 with gaseous HCl leads to the oxidation of the Si 4 6Ϫ Zintl anion and yields nanoporous silicon.

A clathrate containing lithium: K8LixGe44−x/4□2−3x/4 constitutes the first intermetallic clathrate‐I containing lithium. Different to the clathrate phases formed with other alkali metals, Li does not occupy positions within the polyhedral cages but substitutes Ge in the host framework.

The clathrate-I phase Ba(8-x)Si(46) (space group Pm3̅n) was synthesized by oxidation of Ba(4)Li(2)Si(6) with gaseous HCl. Microcrystalline powders of the clathrate phase were obtained within a few minutes. The reaction temperature and the pressure of HCl were optimized to achieve good-quality crystalline products with a composition range of 1.3 < x < 1.9. The new preparation route presented here provides an alternative to the high-pressure synthesis applied so far.