Oxidation of M₄Si₄ (M = Na, K) to Clathrates by HCl or H₂O

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-xSi46 were obtained as crystalline products. The synthetic method demonstrates a mild oxidative route to a large class of metastable intermetallic compounds.