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

Corinna Lorenz,Nikolaus Korber,Stefanie Gärtner

doi:10.3390/cryst8070276

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 The reported solvates Na4Sn4·13NH3, Rb4Sn4·2NH3, Cs4Sn4·2NH3, Rb4Pb4·2NH3 as well as the up to now unpublished crystal structures of the new compounds Cs4Si4·7NH3, Cs4Ge4·9NH3, [Li(NH3)4]4Sn4·4NH3, Na4Sn4·11.5NH3 and Cs4Pb4·5NH3 are considered for comparisons.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.

Highlights

The term “polar intermetallics” applies to a large field of intermetallic compounds, the properties of which range from metallic and superconducting to semiconducting with a real band gap [1,2,3,4,5]
As there is no structural indication for the ammonia molecules to be deprotonated, the [Pb4 ]4− cage is assigned a fourfold negative charge, which is compensated by the four cesium cations
We investigated the relations of ammoniate crystal structures of tetrelide tetrahedranides and the corresponding binary intermetallic phases

Summary

Introduction

The term “polar intermetallics” applies to a large field of intermetallic compounds, the properties of which range from metallic and superconducting to semiconducting with a real band gap [1,2,3,4,5]. 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 salt-like, semiconducting intermetallic compounds which show the presence of the tetrahedral [E4 ]4− anions precast in solid state. These anions are valence isoelectronic to white phosphorus and can be seen as molecular units. Due to coulombic interactions a dense, Crystals 2018, 8, 276; doi:10.3390/cryst8070276 www.mdpi.com/journal/crystals

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Conclusion