Strong Anharmonicity‐Induced Low Thermal Conductivity and High n‐type Mobility in the Topological Insulator Bi_1.1Sb_0.9Te₂S

Abstract

Intrinsically low lattice thermal conductivity (k 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 k lat .Here, we demonstrate that Bi 1.1 Sb 0.9 Te 2 S (BSTS), which is a 3D bulk TI, exhibits ultra-low k lat of 0.46 Wm À 1 K À 1 along with high μ of � 401 cm 2 V À 1 s À 1 . Sound velocity measurements and theoretical calculations suggest that chemical bonding hierarchy and high anharmonicity play a crucial role behind such ultra-low k lat . BSTS possesses low energy optical phonons which strongly couple with the heat carrying acoustic phonons leading to ultra-low k lat . Further, Cl has been doped at the S site of BSTS which increases the electron concentration and reduces the k lat resulting in a promising ntype thermoelectric figure of merit (zT) of � 0.6 at 573 K.

Importance of Four-Phonon Interactions in Lattice Thermal Conductivity and Thermoelectrics: A Case Study

“…Above this carrier concentration, an opposite trend is observed, where | S | increases with an increase in temperature. This is due to bipolar conduction, as observed in other reports . At room temperature (RT), the S value of h-GeS bulk is 538 μVK –1 which decreases to 210 μVK –1 at 900 K. At a higher concentration (>10 20 cm –3 ), the S value increases to 443 μVK –1 at 900 K for n -type materials.…”

Thermoelectric Properties of Single-Phase n-Type Bi14Te13S8

“…8 3 S 1 . 1 7 , and Bi 1.1 Sb 0.9 Te 2 S, 33 which have similar chemical composition to this single-phase compound.…”

“…The Bi 14 Te 13 S 8 phase forms as a secondary phase in multiphase compounds of pseudobinary compounds of Bi 2 Te 3 −Bi 2 S 3 alloys; 30 therefore, a better understanding of the electronic transport properties of this compound as an individual phase can provide insight into the thermoelectric performance of multiphase alloys. Figure 9b compares the zT values of this work with those of nominal composition of Bi 2 Te 2 S 28,30 from the literature, sulfur-alloyed bismuth antimony telluride, 33 and bismuth telluride. 26 The stoichiometric composition of the current study is different from these compounds, therefore, differences in the zT values are expected; nevertheless, the efficiencies are similar to them.…”

“…Figure 9. (a) Figure of merit zT of Bi 2 (Te 1.857 S 1.142 ) 1−x I x (x = 0, 0.005, 0.01, 0.015, and 0.02) between 300 and 520 K, (b) comparison of the figure of merit zT values obtained in this work with Bi 2 Te 1.83 S 1.17 ,26 Bi 2 Te 1.88 S 1.12 ,26 Bi 2 Te 2 S,30 Bi 1.1 Sb 0.9 Te 2 S,33 and Bi 2 Te 2 S.28 …”

“…29 Recent interest in topological insulators has also ignited further studies on this compound due to its enhanced ionic properties derived from a high concentration of sulfur, 32 widely used in Bi 2 Te 3 -based materials to further reduce its thermal conductivity while maintaining high carrier mobility. 33,34 In the structure of Bi 14 Te 13 S 8 , some of the Te atoms of Bi 2 Te 3 are replaced with lighter, more electronegative S atoms, which widen the band gap compared to Bi 2 Te 3 , potentially leading to a higher thermopower. The wider band gap of Bi 14 Te 13 S 8 compared to Bi 2 Te 3 allows for reduction of the bipolar effect which deteriorates the performance of this compound at temperatures higher than 400 K, 35−37 due to a reduction in thermopower.…”