Substitution effects in zintl phases: Synthesis and crystal structure of the novel phases Ae₃Sn_4-xBi_1-x (x ≤ 1; Ae = Sr, Ba) ontaining shubnikov-type nets _∞²[Sn _4-xBi_x]

The compounds Ae₃Sn_4-xBi_1+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) Å, c = 12.859(1) Å for Sr₃Sn _3.36Bi_1.64(3) (1) and a = 9.248(2), c = 13.323(3) Å for Ba₃Sn_3.16Bi_1.84(3) (2). The structure consists of two interpenetrating networks formed by a 3D Ae_6/2Bi substructure (anti-ReO₃ type) forming the host, and layers of interconnected four-member units [Sn_4-xBi_x] 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 Ae₃Sn _4-xBi_1-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.