Ammoniates of Zintl Phases: Similarities and Differences of Binary Phases A4E4 and Their Corresponding Solvates

Abstract

The combination of electropositive alkali metals A (A = Na-Cs) and group 14 elements E (E = Si-Pb) in a stoichiometric ratio of 1:1 in solid state reactions results in the formation of polyanionic salts, which belong to a class of intermetallics for which the term Zintl compounds is used. Crystal structure analysis of these intermetallic phases proved the presence of tetrahedral tetrelide tetraanions [E4]4− precast in solid state, and coulombic interactions account for the formation of a dense, three-dimensional cation-anion network. In addition, it has been shown that [E4]4− polyanions are also present in solutions of liquid ammonia prepared via different synthetic routes. From these solutions crystallize ammoniates of the alkali metal tetrahedranides, which contain ammonia molecules of crystallization, and which can be characterized by X-ray crystallography despite their low thermal stability. The question to be answered is about the structural relations between the analogous compounds in solid state vs. solvate structures, which all include the tetrahedral [E4]4− anions. We here investigate the similarities and differences regarding the coordination spheres of these anions and the resulting cation-anion network. The reported solvates Na4Sn4·13NH3, Rb4Sn4·2NH3, Cs4Sn4·2NH3, Rb4Pb4·2NH3 as well as the up to now unpublished crystal structures of the new compounds Cs4Si4·7NH3, Cs4Ge4·9NH3, [Li(NH3)4]4Sn4·4NH3, Na4Sn4·11.5NH3 and Cs4Pb4·5NH3 are considered for comparisons. Additionally, the influence of the presence of another anion on the overall crystal structure is discussed by using the example of a hydroxide co-crystal which was observed in the new compound K4.5Sn4(OH)0.5∙1.75 NH3.

Keywords

• Zintl compounds
• liquid ammonia
• crystal structure