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 greater than or equal to 650 degreesC. Their structures were determined by single-crystal X-ray diffraction. Rb2Bi8Se13 and Cs2Bi8Se13 are isostructural and crystallize in the monoclinic space group P2(1)/m (No. 11) with a 13.4931(4) Angstrom, b = 4.1558(3) Angstrom, c 24.876(2) Angstrom, beta = 96.571(4)degrees, R1 = 0.0577, and wR2 = 0.1159 [I > 2 sigma (I)] for I and a = 13.704(1) Angstrom, b = 4.1532(4) Angstrom, c = 25.008(2) Angstrom, beta = 96.848(2)degrees, R1 = 0.0497, and wR2 = 0.1123 [I > 2 sigma (I)] for II. CsBi3.67Se6 crystallizes in the orthorhombic space group Pnma (No. 62) with a = 23.421(4) Angstrom, b = 4.1877(8) Angstrom, c 13.710(3) Angstrom, R1 = 0.061.1, and wR2 = 0.1384 [I > 2 sigma (I)]. BaBi2Se4 crystallizes in the hexagonal space group P6(3)/m (No. 176) with a = 26.157(1) Angstrom, c = 4.3245(3) Angstrom, R1 = 0.0371, and wR2 = 0.0817 [I > 2 sigma (I)]. The structure of A(2)Bi(8)Se(13) features a three-dimensional framework consisting of wide rectangular NaC1-type infinite rods, running parallel to the b-axis, which are stitched together by CdI2- and Sb2Se3-type fragments. The NaC1-type blocks are aligned parallel to each other, and between them are rows of alkali metal ions. CsBi3.67Se6 consists Of narrower NaC1-type infinite rods, which share edges. The cesium metal ions reside in the space between these rods. The bismuth sites that connect the NaC1-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-TV exhibit n-type charge transport. Compounds I and II, however, can also exhibit p-type behavior. The thermal conductivity for I and TV is low with room-temperature values of similar to1.6 W/(m .K) for I and similar to1.2 W/(m .K) for TV. The optical band gaps of all compounds range between 0.3 and 0.6 eV.