Transport properties,of Bi2S3 and the ternary bismuth sulfides KBi6.33S10 and K2Bi8S13

We have studied transport properties (resistivity, Hall effect, thermopower, and thermal conductivity) of Bi2S3 and two new ternary bismuth sulfides, KBi6.33S(10) and K2Bi8S13, to explore their potential for thermoelectric applications. While both Bi2S3 and K2Bi8S13 display an essentially metallic conduction, KBi6.33S10 has a distinctly semiconducting character. From the Hall measurements it follows that that dominant carriers in all three compounds are electrons and that the carrier density ih K2Bi8S13 is almost 2 orders of magnitude larger than in Bi2S3 and KBi6.33S10 over the whole temperature range from 4 to 300 K. While the room temperature thermopowers of KBi6.33S10 and K2Bi8S13 are comparable (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 particular the thermal conductivity of KBi6.33S10 is some 25% lower than the thermal conductivity;of Bi2Te3, the benchmark thermoelectric material. To make these materials useful thermoelectrics, ways must be found how to significantly increase their electrical conduction while preserving the reasonably large thermopower and low thermal conductivity.