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A versatile low temperature synthetic route to Zintl phase precursors: Na4Si4, Na4Ge4 and K4Ge4 as examples.

Dalton transactions (Cambridge, England : 2003) (2009-11-19)
Xuchu Ma, Fen Xu, Tonya M Atkins, Andrea M Goforth, Doinita Neiner, Alexandra Navrotsky, Susan M Kauzlarich

Na(4)Si(4) and Na(4)Ge(4) are ideal chemical precursors for inorganic clathrate structures, clusters, and nanocrystals. The monoclinic Zintl phases, Na(4)Si(4) and Na(4)Ge(4), contain isolated homo-tetrahedranide [Si(4)](4-) and [Ge(4)](4-) clusters surrounded by alkali metal cations. In this study, a simple scalable route has been applied to prepare Zintl phases of composition Na(4)Si(4) and Na(4)Ge(4) using the reaction between NaH and Si or Ge at low temperature (420 degrees C for Na(4)Si(4) and 270 degrees C for Na(4)Ge(4)). The method was also applied to K(4)Ge(4), using KH and Ge as raw materials, to show the versatility of this approach. The influence of specific reaction conditions on the purity of these Zintl phases has been studied by controlling five factors: the method of reagent mixing (manual or ball milled), the stoichiometry between raw materials, the reaction temperature, the heating time and the gas flow rate. Moderate ball-milling and excess NaH or KH facilitate the formation of pure Na(4)Si(4), Na(4)Ge(4) or K(4)Ge(4) at 420 degrees C (Na(4)Si(4)) or 270 degrees C (both M(4)Ge(4) compounds, M = Na, K). TG/DSC analysis of the reaction of NaH and Ge indicates that ball milling reduces the temperature for reaction and confirms the formation temperature. This method provides large quantities of high quality Na(4)Si(4) and Na(4)Ge(4) without the need for specialized laboratory equipment, such as Schlenk lines, niobium/tantalum containers, or an arc welder, thereby expanding the accessibility and chemical utility of these phases by making them more convenient to prepare. This new synthetic method may also be extended to lithium-containing Zintl phases (LiH is commercially available) as well as to alkali metal-tetrel Zintl compounds of other compositions, e.g. K(4)Ge(9).

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