Cu₄Bi₄Se₉: A Thermoelectric Symphony of Rattling, Anharmonic Lone-pair, and Structural Complexity

Thermoelectric technology, enabling an environmentally friendly direct heat to electricity conversion, provides a possible alternative energy solution. To obtain a higher thermoelectric conversion efficiency, a larger dimensionless figure of merit ZT is required, which is, however, very difficult owing to the mutually restricted and even reversely correlated key property parameters. Herein, we report for the first time the thermoelectric properties of novel Cu₄Bi₄S₉ and Cu₄Bi₄Se₉ materials with complicated orthorhombic structures. Cu₄Bi₄Se₉ exhibits an extremely low lattice thermal conductivity of about 0.29-0.35 W m^-1 K^-1 that is mainly ascribed to the high lattice anharmonicity coming from the synergistic effect of the crystal structure complexity, soft Cu-Se bonds with lower bonding energy, rattling of the Cu atoms, and the high anharmonicity of Bi atoms carrying stereochemically active lone-pair electrons. In spite of its poor electrical conductivity of 7.33 S cm^-1, Cu₄Bi₄Se₉ realizes a power factor of about 1.37 μW cm^-1 K^-2 at ∼530 K, and a figure of merit, ZT ∼0.24 at ∼530 K. Such a value is comparable with those of the Cu/Ag-Bi/Sb-S/Se-based ternary compounds, particularly, the 19 times higher ZT improvement with respect to the isostructural Cu₄Bi₄S₉ suggests that the enhancing factors mentioned above play significant roles.