Ultralow two-channel thermal conductivity in aikinite

Realizing glass-like ultralow lattice thermal conductivity κL in an ordered crystalline material is overwhelmingly appealing for various applications in thermoelectrics and thermal barrier coatings. Here, based on first-principles calculations, we take CuPbBiS3 as an example and systematically study the underlying physical mechanism of the ultralow and glass-like κL in aikinite. It is found that the strong anharmonic renormalization induces temperature-driven stiffening of the acoustic modes and low-lying optical modes from Pb, Cu, and Bi atoms, while leading to the softening of high-frequency optical phonon modes from S atoms. Further analyses highlight that CuPbBiS3 hosts metavalent bonding, stereochemical lone pair activity, and loosely bonded rattling atoms. These characteristics arouse partially liquid-like state, synergistically contribute to strong anharmonicity, and result in unexpectedly low thermal conductivity. Finally, our calculated κL at 300 K is 0.68 Wm−1K−1 with a non-standard dependency of κL ∝ T−0.7, which reasonably agrees well with experimental values in both magnitude (0.52 ± 0.05 Wm−1K−1 at 300 K) and temperature-dependence trend (∝ T−0.2). We demonstrate that the anomalous thermal transport originates from the dual particle-wave behavior of the heat-carrying phonons, in which wavelike tunneling contribute over 18% to the total κL when T exceeds 300 K. Our work underpins the microscopic origins of ultralow κL in CuPbBiS3, providing crucial insights into designing highly efficient aikinite materials for thermoelectric conversion.