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The n-type sulfur-based material MnBi 4 S 7 exhibits intrinsically low thermal conductivity, which is well explained by double well potential. Se alloying can enlarge the carrier concentration and reduce lattice thermal conductivity, resulting in a peak zT value of 0.31, which reveals a record high value among earth-abundant ternary n-type sulfides. • Systematic investigation of the thermoelectric properties of MnBi 4 S 7-x Se x solid solutions. • Elucidation of low lattice thermal conductivity by first-principle calculations and other advanced experimental techniques. • Potential as competitive n type thermoelectric candidate demonstrated by a peak zT of 0.31. We report self-doped MnBi 4 S 7-x Se 7x solid solution as a novel n-type sulfide thermoelectric material with weak anisotropic transport properties. These compounds feature intrinsically low lattice thermal conductivity of 0.5–0.8 W m−1K−1 in the temperature range of 300–800 K, which is well explained by strong lattice anharmonic vibration, mainly from Bi atoms as verified by theoretical calculations. Intentionally introduced sulfur deficiency acts as an efficient electron donor that enhances the electrical conductivity of MnBi 4 S 7 while alloying with Se can promote electrical conductivity and further reduce thermal conductivity by 25% in the whole temperature region. As a result, the sample of with nominal composition of MnBi 4 S 5.46 Se 1.4 shows a peak zT of ~0.31 at 770 K vertical to the pressing direction. [ABSTRACT FROM AUTHOR]

• Growth of large-sized Ge 1– x In x Bi 2 Te 4 single crystals by Bridgman method. • Ultralow κ L of GeBi 2 Te 4 driven by cation disorder and low phonon velocity. • Resonant level introduced by In-doping greatly improves electrical performance. • Achieving zT ave of 0.36 in the range of 323–573 K for Ge 0.97 In 0.03 Bi 2 Te 4 sample. Resonant level, the impurity level located inside the valence/conduction band, is an effective strategy for decoupling the electrical conductivity and Seebeck coefficient and enhancing the thermoelectric power factor. In this work, we show that In doping induces resonant level and enhances the thermoelectric performance of GeBi 2 Te 4 , a promising n-type thermoelectric material with layered structure. Single crystals grown by Bridgman method were used in this study to take the advantage of its anisotropic transport properties. An ultralow lattice thermal conductivity of ∼0.51 W m−1 K−1 at 323 K was found along the c -axis of GeBi 2 Te 4 , driven by intrinsic cation disorder and the low phonon group velocity from the relatively weak chemical bonds as well as the strong lattice anharmonicity caused by hierarchical bond orders. Both enhanced electrical conductivity and Seebeck coefficient were observed in the In-doped samples, indicating that In-doping not only increases the carrier concentration, but also introduces the resonant level that was further confirmed by the first-principles calculation. A maximum zT of ∼0.42 at 523 K and an average zT of 0.36 over 323 to 573 K were achieved in Ge 0.97 In 0.03 Bi 2 Te 4 crystal sample, ∼100% and ∼89% enhancement compared to those of pristine single crystal sample, respectively. This study demonstrates another example of resonant level for thermoelectric performance enhancement, which may be applied to other related materials. [ABSTRACT FROM AUTHOR]