Pb_mBi₂S_3+m Homologous Series with Low Thermal Conductivity Prepared by the Solution-Based Method as Promising Thermoelectric Materials

Homologous compounds with complex crystal structures are considered as a new class of potential thermoelectric materials due to the possible low thermal conductivity. The thermoelectric properties of PbmBi2S3+m (m = 0, 1, and 3) homologous series are investigated in the temperature range of 300–800 K. The samples are prepared by a solution-based method assisted with EDTA-2Na in aqueous media, followed by sintering at 973 K. All obtained PbmBi2S3+m show an N-type degenerate semiconducting behavior, providing thermoelectric power factors, PF, of 3.7 μW·cm–1·K–2 for PbBi2S4 and 2.5 μW·cm–1·K–2 for Pb3Bi2S6 at 800 K. Meanwhile, very low thermal conductivities, κtotal, ranging from 0.55 to 0.65 and 0.93 to 1.17 W·m–1·K–1 are achieved at 300–800 K for PbBi2S4 and Pb3Bi2S6, respectively. Significantly, both PbBi2S4 and Pb3Bi2S6 reveal extremely low lattice thermal conductivities, κLat, of less than 0.46 and 0.79 W·m–1·K–1 over the entire temperature range, respectively. As a result, the highest thermoelectric figure of merit zT of 0.46 at 800 K is observed in PbBi2S4 due to a higher power factor and lower thermal conductivity compared with Pb3Bi2S6.