Substitution Effects in Zintl Phases: Synthesis and Crystal Structure of the Novel Phases Ae3Sn4−xBi1+x (x ≤ 1; Ae = Sr, Ba) Containing Shubnikov-Type Nets †

The compounds Ae3Sn4−xBi1+x (Ae = Sr, Ba) with x < 1 have been synthesized by solid-state reactions in welded Nb tubes at high temperature. Their structures were determined by single crystal X-ray diffraction studies to be tetragonal; space group I4/mcm (No. 140); Z = 4, with a = 8.968(1) A, c = 12.859(1) A for Sr3Sn3.36Bi1.64(3) (1) and a = 9.248(2), c = 13.323(3) A for Ba3Sn3.16Bi1.84(3) (2). The structure consists of two interpenetrating networks formed by a 3D Ae6/2Bi substructure (anti-ReO3 type) forming the host, and layers of interconnected four-member units [Sn4−xBix] with “butterfly”-like shape as the guest. According to the Zintl-Klemm concept, the compounds are slightly electron deficient and will be charge balanced for x = 1. The electronic structures of Ae3Sn4−xBi1+x calculated by the TB-LMTO-ASA method indicate that the compounds correspond to ideal semiconducting Zintl phases with a narrow band gap for x = 1 (zero-gap semiconductor). The origin of the slight deviation from the optimal electron count for a valance compound is discussed.