Ultralow Lattice Thermal Conductivity in the Aikinite Structure Family, CuXPbXBi2-XS3, and Thermoelectric Properties of Cu0.14Pb0.14Bi1.86S3

In this article, we report the synthesis, characterization, and ultralow thermal conductivity of three complex quaternary chalcogenide compounds, CuxPbxBi2-xS3 [x = 0.14(I), 0.33(II), and 1.0(III)], in the bismuthinite (Bi2S3)-aikinite (CuPbBiS3) solid solution series. All the compounds in this solid solution series can be generated by progressively replacing Bi and a vacancy with Pb and Cu. Compositions in between the end members of the series possess complex disordered crystal structures with mixed occupied Bi/Pb and partially occupied Cu/vacancy sites. Density functional theory (DFT)-based phonon calculations suggest that these intrinsic structural attributes and presence of interstitial Cu lead to ultralow thermal conductivities less than 0.65 W m-1 K-1 between 300 and 700 K. All the compounds exhibit a narrow band gap below 1.0 eV confirmed by diffuse reflectance spectroscopy. DFT-based band structure analysis shows that the band gap decreases with the increasing amount of Cu and Pb substitution due to increased contribution of the Cu d states to the valence band. High-temperature charge transport properties indicate that electrons are the dominant charge carriers resulting in low electrical conductivities and relatively large Seebeck coefficients. A promising figure of merit, zT, of 0.21 for I has been achieved at 475 K.

Automated summary

In this article, the synthesis, characterization, and ultralow thermal conductivity of three complex quaternary chalcogenide compounds were reported. These compounds possess complex structures with mixed occupied sites, leading to ultralow thermal conductivities. Spectroscopy and charge transport properties reveal narrow band gaps, low electrical conductivities, and high Seebeck coefficients. One of the compounds exhibits a promising figure of merit at high temperatures.