The First Quaternary Bismuthide Zintl Phase Ba8Zn2-xIn3+xBi10 with complex Crystal Structure

The first quaternary Zintl phase Ba8Zn2–xIn3+xBi10 (x = 0.21–0.47) was synthesized by reacting the elements at high temperature and, its crystal structure determined from single crystal X-ray diffraction data. It crystallizes in the monoclinic space group C2/m, with a large unit cell volume, V = 3351.3(6) Å3 (Z = 4), which is a new structure type. The basic structural building blocks are MBi4 tetrahedra (M = In or Zn/In mix) that are condensed by sharing corners when mainly centered by In atom, or corners and edges when mainly centered by Zn atom. In turn, these tetrahedra form two markedly distinct one-dimensional (1-D) polyanionic substructures, defining two virtually distinct sub-structures. Corollary, the title structure can be viewed as an intergrowth of two imaginary quaternary phases (1A) “Ba3Zn1–yIn1+yBi4” and (1B) “Ba5Zn1–zIn2+zBi6” (with x = y+z). The polyanionic system in the 1A block, [Zn1–yIn1+yBi4]6– consists of dimeric pentagonal rings that are stacked to form double pentagonal tubes running in the b-direction, and filled with Ba cations. The polyanion in the 1B block, [Zn1–zIn2+zBi6]6– consists of large ribbons formed by the condensation of two strands of dimeric corner sharing infinite chains of [InBi4] tetrahedra with one chain of edge-sharing [ZnBi4] tetrahedra. The Zn/In atomic site preferences appeared to be one decisive factor behind the structural complexity and, it seems to be directed mainly by the atomic valence states and sizes, as well as the electrostatic factors, aiming to optimize the lattice reticular energy, as confirmed by total energy calculations on three ordered models using DFT methods. According to the Zintl-Klemm concept (ZKC) and the 8–N rules, the compound should be charge balanced for x = 0. Therefore, the experimentally observed phase width (x = 0.21 to 0.47) indicates that the system is amenable for substantial (n- or p-type) doping, to fine tune their transport properties, like the charge carrier concentration and charge mobility, via specific chemical substitutions at specific sites of the two polyanionic blocks. The DFT electronic band structure calculations on the hypothetical fully ordered model ‘Ba8Zn2In3Bi10’ (x = 0), also predicted the title compound to be a semiconductor with narrow band gap. Therefore, the title phase fully qualified as a promising thermoelectric material candidate.

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