Rb 2 Bi 8 Se 13 (I), Cs 2 Bi 8 Se 13 (II), CsBi 3.67 Se 6 (III), and BaBi 2 Se 4 (IV) were synthesized by direct combination reactions of the A/Se (A ) Rb, Cs, Ba) and Bi 2 Se 3 at g650 °C. Their structures were determined by single-crystal X-ray diffraction. Rb 2 Bi 8 Se 13 and Cs 2 Bi 8 Se 13 are isostructural and crystallize in the monoclinic space group P2 1 /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. CsBi 3.67 Se 6 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)]. BaBi 2 Se 4 crystallizes in the hexagonal space group P6 3 /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 A 2 Bi 8 Se 13 features a three-dimensional framework consisting of wide rectangular NaCl-type infinite rods, running parallel to the b-axis, which are stitched together by CdI 2 -and Sb 2 Se 3 -type fragments. The NaCl-type blocks are aligned parallel to each other, and between them are rows of alkali metal ions. CsBi 3.67 Se 6 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 [Bi 2 Se 4 ] 2framework in BaBi 2 Se 4 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.
