SnTe, as tructural analogue of champion thermoelectric (TE) material PbTe, has recently attracted wide attention for TE energy conversion. Herein, we demonstrate ac o-doping strategy to improve the TE performance of SnTe via simultaneous modulation of electronic structure and phonon transport. The electrical transport is optimizedb y 3mol %A gd oping in self-compensated SnTe( i.e., Sn 1.03 Te). Further,Mgdoping in SnAg 0.03 Te resulted in highly converged valence bands,w hich enhanced the Seebeck coefficient markedly.T he energy gap between two uppermost valence bands (DE v )decreases to 0.10 eV in Sn 0.92 Ag 0.03 Mg 0.08 Te compared to 0.35 eV in pristine SnTe. The optimizedp-type carrier concentration and highly converged valence bands gave ahigh power factor of ca. 27 mWcm À1 K À2 at 865 Ki nS n 0.92 Ag 0.03 Mg 0.08 Te. The lattice thermal conductivity of Sn 0.92 Ag 0.03 Mg 0.08 Te reached to an ultra-low value of % 0.23 Wm À1 K À1 at 865 Kd ue to the formation of MgTen anoprecipitates in SnTem atrix. These combined effects resulted in ah igh TE figure of merit, zT % 1.55 at 865 Ki nSn 0.92 Ag 0.03 Mg 0.08 Te.

High thermoelectric performance is generally achieved in solid-solution alloyed or heavily doped semiconductors. The consequent atomic disorder has a trade-off in the thermoelectric figure of merit, zT: lattice thermal conductivity...

SnTe, as tructural analogue of champion thermoelectric (TE) material PbTe, has recently attracted wide attention for TE energy conversion. Herein, we demonstrate ac o-doping strategy to improve the TE performance of SnTe via simultaneous modulation of electronic structure and phonon transport. The electrical transport is optimizedb y 3mol %A gd oping in self-compensated SnTe( i.e., Sn 1.03 Te). Further,Mgdoping in SnAg 0.03 Te resulted in highly converged valence bands,w hich enhanced the Seebeck coefficient markedly.T he energy gap between two uppermost valence bands (DE v )decreases to 0.10 eV in Sn 0.92 Ag 0.03 Mg 0.08 Te compared to 0.35 eV in pristine SnTe. The optimizedp-type carrier concentration and highly converged valence bands gave ahigh power factor of ca. 27 mWcm À1 K À2 at 865 Ki nS n 0.92 Ag 0.03 Mg 0.08 Te. The lattice thermal conductivity of Sn 0.92 Ag 0.03 Mg 0.08 Te reached to an ultra-low value of % 0.23 Wm À1 K À1 at 865 Kd ue to the formation of MgTen anoprecipitates in SnTem atrix. These combined effects resulted in ah igh TE figure of merit, zT % 1.55 at 865 Ki nSn 0.92 Ag 0.03 Mg 0.08 Te.

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.

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.