Abstract/Summary

Understanding the mechanism that connects heat transport with crystal structures and order/disorder phenomena is crucial to develop materials with ultra-low thermal conductivity (K), for thermoelectric and thermal barriers applications and requires the study of highly pure materials. We synthesized the n-type sulfide CuPbBi5S9 with ultra-low K of 0.4-0.6 in the temperature range 300-700 K. In contrast to prior studies, we show that this synthetic sulfide is highly pure and does not exhibit the ordered gladite mineral structure but instead a copper-deficient partially disordered aikinite structure with Bi replacing Pb, according to the chemical formula Cu1/3□2/3Pb1/3Bi5/3S3. By combining experiments and lattice dynamics calculations, we elucidated that the ultra-low K of this compound is a result of combined structural disorder induced by the processing method and very low energy optical modes associated with Pb and Bi ions and, to a smaller extent, Cu. This vibrational complexity at low energy hints to substantial anharmonic effects that contributes to enhance phonon scattering. Importantly, we show that this aikinite sulfide, despite being a poor semiconductor, is a potential matrix for designing novel efficient n-type thermoelectric compounds with ultralow K. A drastic improvement of the carrier concentration and thermoelectric figure of merit have been obtained upon Cl for S and Bi for Pb substitution. The series Cu1-x□xPb1-xBi1+xS3 provide an interesting structural prototype for engineering n-type thermoelectric sulfides by controlling disorder and optimizing doping.