A2Bi8Se13 (A = Rb, Cs), CsBi3.67Se6, and BaBi2Se4:  New Ternary Semiconducting Bismuth Selenides

Rb2Bi8Se13 (I ), Cs2Bi8Se13 ( II ), CsBi3.67Se6 ( III ), and BaBi2Se4 (IV) were synthesized by direct combination reactions of the A/Se (A = Rb, Cs, Ba) and Bi2Se3 at ≥650 °C. Their structures were determined by single-crystal X-ray diffraction. Rb2Bi8Se13 and Cs2Bi8Se13 are isostructural and crystallize in the monoclinic space group P21 /m (No. 11) with a = 13.4931(4) Å, b = 4.1558(3) Å, c = 24.876(2) Å, β = 96.571(4)°, R1 = 0.0577, and wR2 = 0.1159 [I > 2σ(I)] for I and a = 13.704(1) Å, b = 4.1532(4) Å, c = 25.008(2) Å, β = 96.848(2)°, R1 = 0.0497, and wR2 = 0.1123 [I > 2σ(I)] for II. CsBi3.67Se6 crystallizes in the orthorhombic space group Pnma (No. 62) with a = 23.421(4) Å, b = 4.1877(8) Å, c = 13.710(3) Å, R1 = 0.0611, and wR2 = 0.1384 [I > 2σ(I)]. BaBi2Se4 crystallizes in the hexagonal space group P63 /m (No. 176) with a = 26.157(1) Å, c = 4.3245(3) Å, R1 = 0.0371, and wR2 = 0.0817 [I > 2σ(I)]. The structure of A2Bi8Se13 features a three-dimensional framework consisting of wide rectangular NaCl-type infinite rods, running parallel to the b-axis, which are stitched together by CdI2- and Sb2Se3-type fragments. The NaCl-type blocks are aligned parallel to each other, and between them are rows of alkali metal ions. CsBi3.67Se6 consists of narrower NaCl-type infinite rods, which share edges. The cesium metal ions reside in the space between these rods. The bismuth sites that connect the NaCl-type rods are partially occupied. The [Bi2Se4]2- framework in BaBi2Se4 contains tunnels running along the c-axis that are occupied by Ba atoms. All compounds are narrow band-gap semiconductors. Electrical conductivity and thermoelectric power measurements show that I − IV exhibit n-type charge transport. Compounds I and II, however, can also exhibit p-type behavior. The thermal conductivity for I and IV is low with room-temperature values of ∼1.6 W/(m·K) for I and ∼1.2 W/(m·K) for IV. The optical band gaps of all compounds range between 0.3 and 0.6 eV.