Ultralow Thermal Conductivity in Earth-Abundant Cu1.6Bi4.8S8: Anharmonic Rattling of Interstitial Cu

Abstract

Earth-abundant, nontoxic crystalline compounds with intrinsically low lattice thermal conductivity (κlat) are centric to the development of thermoelectrics and thermal barrier coatings. Investigation of the fundamental origins of such low κlat and understanding its relationship with the chemical bonding and structure in solids thus stands paramount in order to furnish such low thermally conductive compounds. Herein, we synthesized earth-abundant, cost-effective, and nontoxic n-type ternary sulfide Cu1.6Bi4.8S8, which exhibits an intrinsically ultralow κlat of ∼0.71–0.44 W/m·K in the temperature range of 296–736 K. Structural analysis via atomic refinement unveiled large atomic displacement parameters (ADPs) for interstitial Cu clusters, demonstrating intrinsic rattling-like behavior. Electron localization function (ELF) analysis further shows that these rattling Cu atoms are weakly bonded and thus can generate low-energy Einstein vibrational modes. Low-temperature heat capacity (Cp) and temperature-dependent Raman spectra concord the presence of such low-energy optical modes. Density functional theory (DFT)-based phonon dispersions reveal that these low-lying optical phonons arise primarily due to the presence of chemical bonding hierarchy and simultaneous rattling of weakly bonded interstitial Cu atoms. These low-energy optical modes strongly scatter the heat-carrying acoustic phonons, thereby reducing the phonon lifetime to an ultrashort value (2–4.5 ps) and κlat to a very low value, which is lower than that of the many state-of-the-art metal sulfides.