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

Abstract

Na₄Si₄ and Na₄Ge₄ are ideal chemical precursors for inorganic clathrate structures, clusters, and nanocrystals. The monoclinic Zintl phases, Na₄Si₄ and Na₄Ge₄, contain isolated homo-tetrahedranide [Si₄]⁴⁻ and [Ge₄]⁴⁻ clusters surrounded by alkali metal cations. In this study, a simple scalable route has been applied to prepare Zintl phases of composition Na₄Si₄ and Na₄Ge₄ using the reaction between NaH and Si or Ge at low temperature (420 °C for Na₄Si₄ and 270 °C for Na₄Ge₄). The method was also applied to K₄Ge₄, 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 Na₄Si₄, Na₄Ge₄ or K₄Ge₄ at 420 °C (Na₄Si₄) or 270 °C (both M₄Ge₄ 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 Na₄Si₄ and Na₄Ge₄ 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. K₄Ge₉.