Electronic Structure Analysis of the A 10 Tt 2 P 6 System (A=Li-Cs; Tt=Si, Ge, Sn) and Synthesis of the Direct Band Gap Semiconductor K 10 Sn 2 P 6 .

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 A 10 Tt 2 P 6 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 TtP 3 5- units which are isostructural to CO 3 2- or [Tt 2 P 6 ] 10- dimers which correspond to two edge-sharing TtP 4 tetrahedra. The A 10 Tt 2 P 6 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 Na 10 Si 2 P 6 , Na 10 Ge 2 P 6 and K 10 Sn 2 P 6 were found to be candidates for a high temperature phase transition between the two polymorphs. Additionally, the unknown, but predicted compound K 10 Sn 2 P 6 was synthesized and characterized by single crystal and powder x-ray diffraction. It crystalizes in the monoclinic space group P 2 1 /n and incorporates [Sn 2 P 6 ] 10- edge sharing double tetrahedra. It was determined to be a direct band gap semiconductor with a band gap of 2.57 eV.