Preparation and thermoelectric power of SnBi₄Se₇

Polycrystalline samples of Bi2Se3 and a stoichiometric ternary compound in the quasi-binary system SnSe–Bi2Se3 have been prepared and characterized by X-ray powder diffraction analysis. At room temperature the carrier concentration values are n = 1.1 × 1019 cm−3 for Bi2Se3 and n = 0.53 × 1019 cm−3 for SnBi4Se7. The thermoelectric power has been measured over the temperature range 90–420 K. The thermoelectric power of Bi2Se3 is higher than that for SnBi4Se7, which shows that the Sn impurity has an acceptor character. Therefore, doping Bi2Se3 with tin atoms does not improve thermoelectric properties of this material, due to decrease the value of the power factor σS 2. Transport properties of the studied polycrystalline samples are characterized by a mixed transport mechanism of free carriers. It is necessary to add more than one Sn atom to the Bi2Se3 compound in order to suppress the electron concentration by one electron. Such behaviour of the dopant is explained by the formation of various structural defects. Besides the dominant substitutional defect, SnBi, tin atoms also form uncharged defects, corresponding to seven-layer lamellae of the composition Se–Bi–Se–Sn–Se–Bi–Se which corresponds to the structure of the SnBi2Se4.