Electronic Structure Analysis of the A₁₀Tt₂P₆ System (A=Li−Cs; Tt=Si, Ge, Sn) and Synthesis of the Direct Band Gap Semiconductor K₁₀Sn₂P₆

Investigating the relationship between atomic and electronic structures is a powerful tool to screen the wide variety of Zintl phases for interesting (opto−)electronic properties. To get an insight in such relations, the A₁₀Tt₂P₆ system (A=Li−Cs; Tt=Si−Sn) was picked as model system to analyse the influence of structural motives, combination of elements and their properties on type and width of the band gaps. Those compounds comprise two interesting structural motives of their anions, which are either monomeric trigonal planar TtP₃⁵⁻ units which are isostructural to CO₃²⁻ or [Tt₂P₆]¹⁰⁻ dimers which correspond to two edge-sharing TtP₄ tetrahedra. The A₁₀Tt₂P₆ compounds were structurally optimized for both polymorphs and subsequent frequency analysis, band structure as well as density of states calculations were performed. The Gibbs free energies were compared to determine temperature dependent stability, where Na₁₀Si₂P₆, Na₁₀Ge₂P₆ and K₁₀Sn₂P₆ were found to be candidates for a high temperature phase transition between the two polymorphs. Additionally, the unknown, but predicted compound K₁₀Sn₂P₆ was synthesized and characterized by single crystal and powder x-ray diffraction. It crystalizes in the monoclinic space group P 2₁/n and incorporates [Sn₂P₆]¹⁰⁻ edge sharing double tetrahedra. It was determined to be a direct band gap semiconductor with a band gap of 2.57 eV.