Understanding the structural and physical origins of low thermal conductivity behavior is essential for improving and searching for high-efficiency thermoelectric materials. Natural minerals are cheap and usually have low thermal conductivities. The lattice thermal conductivities of two isostructural natural materials, chalcostibite ${\mathrm{CuSbS}}_2$ and emplectite ${\mathrm{CuBiS}}_2$, are substantially low in experimental measurements. In particular, the lattice thermal conductivity of ${\mathrm{CuBiS}}_2$ is much lower than that of ${\mathrm{CuSbS}}_2$. Using first-principles Debye-Callaway calculations, we found that the lattice thermal conductivities of ${\mathrm{CuSbS}}_2$ and ${\mathrm{CuBiS}}_2$ are 1.44 W/mK and 0.46 W/mK at 300 K, respectively, which are in good agreement with the experimental measurements. From the calculated vibrational properties, we demonstrate that the stereochemically active lone-pair electrons at the Sb sites are major contributors to the low thermal conductivity of ${\mathrm{CuSbS}}_2$. However, for ${\mathrm{CuBiS}}_2$, the dual effects of the lone-pair electrons at the Bi sites and the rattling of the Cu ions are the primary reasons for the ultralow thermal conductivity. Because of the ultralow thermal conductivity in ${\mathrm{CuBiS}}_2$, our predicted highest $ZT$ value in the material could reach 0.91 for $n$-type doping at 700 K and 0.77 for $p$-type doping at 780 K, which implies that ${\mathrm{CuBiS}}_2$ can be utilized as a potential low-cost thermoelectric material for both $n$ and $p$ type. The present work emphasizes the importance of lone-pair electrons and rattling modes in impelling the phonon anharmonicity, providing a useful guide to seek and design new thermoelectric materials with ultralow thermal conductivity and high efficiency.

• Received 28 September 2017

DOI:https://doi.org/10.1103/PhysRevB.96.235205