TRANSPORT PROPERTIES,OF BI2S3 AND THE TERNARY BISMUTH SULFIDES KBI6.33S10 AND K2BI8S13

We have studied transport properties (resistivity, Hall effect, thermo power, and thermal conductivity) of Bi2S3 and two new ternary bismuth sulfides, KBi6.33S(10) and K2Bi8S13, to explore their potential for th ermoelectric applications. While both Bi2S3 and K2Bi8S13 display an es sentially metallic conduction, KBi6.33S10 has a distinctly semiconduct ing character. From the Hall measurements it follows that that dominan t carriers in all three compounds are electrons and that the carrier d ensity ih K2Bi8S13 is almost 2 orders of magnitude larger than in Bi2S 3 and KBi6.33S10 over the whole temperature range from 4 to 300 K. Whi le the room temperature thermopowers of KBi6.33S10 and K2Bi8S13 are co mparable (similar to-100 mu V/K), the thermopower of Bi2S3 is a factor of 3 larger. All three compounds are poor conductors of heat, in part icular the thermal conductivity of KBi6.33S10 is some 25% lower than t he thermal conductivity;of Bi2Te3, the benchmark thermoelectric materi al. To make these materials useful thermoelectrics, ways must be found how to significantly increase their electrical conduction while prese rving the reasonably large thermopower and low thermal conductivity.