Strong Anharmonicity‐Induced Low Thermal Conductivity and High n‐type Mobility in the Topological Insulator Bi<sub>1.1</sub>Sb<sub>0.9</sub>Te<sub>2</sub>S
Riddhimoy Pathak, Prabir Dutta, Ashutosh Srivastava, Divya Rawat, Radha Krishna Gopal, Abhishek K. Singh, Ajay Soni, Kanishka Biswas
Abstract
Abstract 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 Te 2 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 cm 2 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.