Ultralow Lattice Thermal Conductivity in the Aikinite Structure Family, Cu_xPb_xBi_2–xS₃, and Thermoelectric Properties of Cu_0.14Pb_0.14Bi_1.86S₃

Abstract

In this article, we report the synthesis, characterization, and ultralow thermal conductivity of three complex quaternary chalcogenide compounds, Cu x Pb x Bi 2−x S 3 [x = 0.14(I), 0.33(II), and 1.0(III)], in the bismuthinite (Bi 2 S 3 )−aikinite (CuPbBiS 3 ) 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.

Lone Pair Rotation and Bond Heterogeneity Leading to Ultralow Thermal Conductivity in Aikinite

“…Moreover, based on the limited data available in the literature, members of this series with x < 1 exhibit similarly low values of thermal conductivity to those of aikinite. This ultralow thermal conductivity has previously been ascribed to the complex crystal structure with a large unit cell and heavy atoms, or to disorder . However, given that aikinite has an ultralow thermal conductivity even with complete ordering of the Cu + , Pb 2+ , and Bi 3+ cations, we can effectively discount disorder as the origin of the ultralow thermal conductivity in other members of the Cu 1– x □ x Pb 1– x Bi 1+ x S 3 series with x < 1.…”

“…This ultralow thermal conductivity has previously been ascribed to the complex crystal structure with a large unit cell and heavy atoms, 32 or to disorder. 78 However, given that aikinite has an ultralow thermal conductivity even with complete ordering of the Cu + , Pb 2+ , and Bi 3+ cations, we can effectively discount disorder as the origin of the ultralow thermal conductivity in other members of the Cu 1−x □ x Pb 1−x Bi 1+x S 3 series with x < 1. Furthermore, the larger thermal conductivity of Bi 2 S 3 (x = 1) when compared to other members of the series indicates that the Cu + and Pb 2+ cations play a key role in heat transport.…”