Syntheses, crystal Structures and electronic Structures of new metal chalcoiodides Bi₂CuSe₃I and Bi₆Cu₃S₁₀I

Two new metal chalcoiodides were synthesized by solid-state reactions at 400 °C. Crystal Data: Bi₂CuSe₃I, 1, monoclinic, C2/m, a=14.243(2) Å, b=4.1937(7) Å, c=14.647(2) Å, β=116.095(2)°, V=785.7(2) ų, and Z=4; Bi₆Cu₃S₁₀I, 2, orthorhombic, Pnma, a=17.476(2) Å, b=4.0078(4) Å, c=27.391(2) Å, V=1918.5(3) ų, and Z=4. Compound 1 adopts a three-dimensional structure formed by two alternative layers, which consist of BiSe₅ square pyramids, BiSe₄I₂ octahedra, CuSe₄ tetrahedra, and CuSe₂I₂ tetrahedra. Compound 2 possesses a new open framework built up of BiS₅ square pyramides, BiS₆ octahedra, BiS₈ polyhedra, and CuS₄ tetrahedra where I^- anions are uniquely trapped within the tunnels. Both electronic structures reveal that bismuth and chalcogenide orbitals dominate the bandgaps. The Cu d and I p states contribute to the top of valence bands, in which the distribution of I orbitals may correspond to the relative bonding interactions in 1 and 2. The optical bandgaps determined by the diffuse reflectance spectra are 0.68 eV and 0.72 eV for 1 and 2, respectively. 1 is a p-type semiconductor with high Seebeck coefficients of 460-575 μV/K in the temperature range of 300-425 K. The electrical conductivity is 0.02 S/cm at 425 K for the undoped sample. The thermal conductivity is 0.22 W/mK at 425 K.