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

Abstract

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.

Synthesis of the intermetallic clathrate Na2Ba6Si46 by oxidation of Na2BaSi4 with HCl

“…More recently, the oxidation process has turned out to be notably simplified by gas-solid reactions, and the clathrate-I silicides Na 6.2 Si 46 and K 7.0 Si 46 could be obtained by oxidation of NaSi and KSi with HCl [9]. Here we report on the oxidation of Na 2 BaSi 4 with gaseous HCl to Na 2 Ba 6 Si 46 with fully occupied metal sites.…”

“…Gaseous NH 3 and H 2 have no significant influence on the oxidation of the precursor (see also [9]). …”

“…The preparation of Na 2 Ba 6 Si 46 follows a method that was recently reported for the syntheses of Na 6.2 Si 46 and K 7.0 Si 46 [9]. The precursor Na 2 BaSi 4 was reacted in a closed glass reactor with gaseous HCl to Na 2 Ba 6 Si 46 and metal chlorides:…”

Cs8−xSi46: A Type‐I Clathrate with Expanded Silicon Framework

“…[15] Accordingly, Na and K atoms fit into both kinds of cages while Rb atoms fit only into the larger Si 24 cages. These clathrates have actually been obtained by thermal decomposition [15] or oxidation [16] of the monosilicides M 4 Si 4 (M = Na, K, Rb). [17a,b] The radius of Cs atoms was estimated to be too large for both types of cages in the Si 46 framework and, therefore, the formation of the clathrate-II silicide Cs~7Si 136 is attributed to the preferential occupation of the largest Si 28 cages in this atomic arrangement.…”

K7B7Si39, a Borosilicide with the Clathrate I Structure

“…[5] K 7 B 7 Si 39 is the first borosilicide with a clathrate I crystal structure, and it has the smallest lattice parameter among all known clathrate compounds based on Group 14 elements. The compound can be obtained in the form of well-shaped crystals, [6] which is unusual, since the silicon clathrates are normally obtained as fine polycrystalline powders by thermal decomposition [1] or oxidation [7] of reactive starting compounds. Single crystals were prepared in only a few cases.…”