Molten Salt Synthesis and Properties of Three New Solid-State Ternary Bismuth Chalcogenides, β-CsBiS2, γ-CsBiS2, and K2Bi8Se13

Timothy J. McCarthy, Stanley Pierre Ngeyi, Ju Hsiou Liao, Donald C. DeGroot, Tim Hogan, Carl R. Kannewurf, Mercouri G. Kanatzidis

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Abstract

β-CsBiS2, γ-CsBiS2, and K2Bi8Se13 were synthesized by the alkali metal polychalcogenide flux technique. An investigation of the Bi/Cs2Sx system at 290 °C revealed the presence of a new ternary sulfide β-CsBiS2 (I) (83% yield). The red substance crystallizes in the monoclinic P21/c space group (No. 14) with a = 7.794(5) Å, b = 9.610(6) Å, c = 7.329(4) Å, β = 102.16(5)°. The final R/Rw = 7.4/10%. The structure consists of BiS3 trigonal pyramids connected at two vertices to form (BiS2)nn− chains along the a axis with Cs+ ions in between the chains. β-CsBiS2 is an insulator. γ-CsBiS2 was obtained as above but at 350 °C. It crystallizes in the rhombohedral system with a = 4.166(4) Å and c = 48.77(5) Å. It has a superstructure related to that of RbBiS2. Similar investigations in the Bi/K2Sex system at 330 °C gave the new ternary selenide K2Bi8Se13 (II) (88% yield). This compound crystallizes in the triclinic P1̄ space group (No. 2) with a = 13.768(2) A, b = 12.093(3) Å, c = 4.1656(6) Å, α = 89.98(1)°, β = 98.64(1)°, γ = 87.96(1)°. The final R/Rw = 8.2/8.9%. The structure of the black crystals consists of layers of BiSe6 octahedra with BiSe5 square pyramidal units connecting the layers to form a three-dimensional network with K+ ions located in the channels. The [Bi8Se13]2− framework can be thought of as a hybrid of three different layered structure types interconnected to form a 3-D network. Structural features from the Bi2Te3, Sb2S3, and CdI2 lattices are represented in this framework. Electrical conductivity and thermopower measurements on γ-CsBiS2 and K2Bi8Se13 show n-type semiconducting behavior. For both materials the room temperature conductivity is ∼ 10−2 S/cm. The reaction of Bi with Cs2Sx and K2Sex fluxes was investigated with differential scanning calorimetry (DSC). The optical band gaps of β-CsBiS2, γ-CsBiS2, and K2Bi8Se13 were determined by UV/vis/near-IR diffuse reflectance measurements to be 1.43, 1.11, and 0.76 eV, respectively.

Original languageEnglish
Pages (from-to)331-340
Number of pages10
JournalChemistry of Materials
Volume5
Issue number3
DOIs
Publication statusPublished - Jan 1 1993

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ASJC Scopus subject areas

  • Chemistry(all)
  • Chemical Engineering(all)
  • Materials Chemistry

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