Open Sn Framework Structure Hosting Bi Guest atoms-Synthesis, Crystal and Electronic Structure of Na₁₃Sn₂₆Bi

The large variety of structures of Zintl phases are generally well understood since their anionic substructures follow bonding rules according to the valence concept. But there are also exceptions, which make the semiconductors especially interesting in terms of structure-property relationships. Although several Na-Sn-Pnictides with a variety of structural motives are known, up to this point no ternary compound in the Na-Sn-Bi system has been described. In this paper we present the Zintl-phase Na₁₃Sn_25.73Bi_1.27 comprising a complex, open-framework structure of Sn atoms, with one mixed Sn/Bi site, hosting Na atoms. An additional Bi atom is loosely connected with only weak contacts to the framework filling a larger cavity within the network. According to band structure calculations of the two ordered variants with either full occupation of the mixed site with Sn or Bi, resulting in Na₁₃Sn₂₆Bi and Na₁₃Sn₂₄Bi₃, respectively, both compounds are semiconductors with band gaps of 0.5 eV. A comparison of the band structures with the structurally related binary compounds Na₅Sn₁₃ and Na₇Sn₁₂ shows that only the perfectly charge balanced Na₇Sn₁₂ is a semiconductor whereas Na₅Sn₁₃ is metallic. The rather specific electronic situation in the ternary compound is traced back to the loosely bound Bi atom, which acts as a guest atom according to Bi_x@Na₁₃Sn_26-yBi_y, with x=1 and y=0.27, capable to change its oxidation state and thus to uptake additional electrons allowing the system to be a semiconductor. Therefore, Na₁₃Sn_25.73Bi_1.27 can be understood as a rare example of an open framework structure of Sn atoms comprising Bi atoms in the cavities.