Strong Anharmonicity-Induced Low Thermal Conductivity and High n-type Mobility in the Topological Insulator Bi_1.1...

Intrinsically low lattice thermal conductivity (κ_lat ) while maintaining the high carrier mobility (μ) is of the utmost importance for thermoelectrics. Topological insulators (TI) can possess high μ due to the metallic surface states. TIs with heavy constituents and layered structure can give rise to high anharmonicity and are expected to show low κ_lat . Here, we demonstrate that Bi_1.1 Sb_0.9 Te₂ S (BSTS), which is a 3D bulk TI, exhibits ultra-low κ_lat of 0.46 Wm^-1  K^-1 along with high μ of ≈401 cm²  V^-1  s^-1 . Sound velocity measurements and theoretical calculations suggest that chemical bonding hierarchy and high anharmonicity play a crucial role behind such ultra-low κ_lat . BSTS possesses low energy optical phonons which strongly couple with the heat carrying acoustic phonons leading to ultra-low κ_lat . Further, Cl has been doped at the S site of BSTS which increases the electron concentration and reduces the κ_lat resulting in a promising n-type thermoelectric figure of merit (zT) of ≈0.6 at 573 K.