Strong Anharmonicity‐Induced Low Thermal Conductivity and High n‐type Mobility in the Topological Insulator Bi 1.1 Sb 0.9 Te 2 S

Abstract

Intrinsically low lattice thermal conductivity (κlat) 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 κlat. Here, we demonstrate that Bi1.1Sb0.9Te2S (BSTS), which is a 3D bulk TI, exhibits ultra-low κlat 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 κlat. BSTS possesses low energy optical phonons which strongly couple with the heat carrying acoustic phonons leading to ultra-low κlat. Further, Cl has been doped at the S site of BSTS which increases the electron concentration and reduces the κlat resulting in a promising n-type thermoelectric figure of merit (zT) of ≈0.6 at 573 K.