Transport Properties of Bi₂S₃ and the Ternary Bismuth Sulfides KBi_6.33S₁₀ and K₂Bi₈S₁₃

We have studied transport properties (resistivity, Hall effect, thermopower, and thermal conductivity) of Bi₂S₃ and two new ternary bismuth sulfides, KBi_6.33S₁₀ and K₂Bi₈S₁₃, to explore their potential for thermoelectric applications. While both Bi₂S₃ and K₂Bi₈S₁₃ display an essentially metallic conduction, KBi_6.33S₁₀ has a distinctly semiconducting character. From the Hall measurements it follows that the dominant carriers in all three compounds are electrons and that the carrier density in K₂Bi₈S₁₃ is almost 2 orders of magnitude larger than in Bi₂S₃ and KBi_6.33S₁₀ over the whole temperature range from 4 to 300 K. While the room temperature thermopowers of KBi_6.33S₁₀ and K₂Bi₈S₁₃ are comparable (∼-100 μV/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 KBi_6.33S₁₀ 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.