Dual effects of lone-pair electrons and rattling atoms in CuBiS2 on its ultralow thermal conductivity

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.