AgBi₃S₅ is a new n-type thermoelectric material that is environmentally friendly and composed of elements of earth-abundant, non-toxic and high performance-cost ratio. This compound features an intrinsically low thermal conductivity derived from its complex monoclinic structure. However, the terrible electrical transport properties greatly limited the improvement of thermoelectric performance. Most previous studies considered that carrier concentration is the main reason for low electrical conductivity and focused on improving carrier concentration by aliovalent ion doping. In this work, we found that the critical parameter that restricts the electric transport performance of AgBi₃S₅ was the extremely low carrier mobility instead of the carrier concentration. According to the Pisarenko relationships and density functional theory calculations, Nb doping can sharpen the conduction band of AgBi₃S₅, which contributes to reducing the effective mass and improving the carrier mobility. With a further increase of the Nb doping content, the conduction band convergence can enlarge the effective mass and preserve the carrier mobility. Combined with the decrease in lattice thermal conductivity due to the intensive phone scattering, a maximum ZT value of ~0.50 at 773 K was achieved in Ag_0.97Nb_0.03Bi₃S₅, which was ~109.6% higher than that of pure AgBi₃S₅. This work will stimulate the new exploration of high-performance thermoelectric materials in ternary metal sulfides.

The wide-bandgap cubic-structure semiconductor In₄SnSe₄ can be regarded as a product of compositing two typical layered thermoelectric materials SnSe and In₄Se₃. Remarkably, In₄SnSe₄ inherited low thermal conductivity from its parent materials. To advance the potential thermoelectric property of In₄SnSe₄, we systematically investigated its crystal structure and the origin of the intrinsic low thermal conductivity. In₄SnSe₄ crystallized in a cubic phase (space group pa3), with the lattice parameters of a = b = c = 12.66 Å. The anisotropy of InSe bonds in the lattice determined the complex structure of In₄SnSe₄ with 72 atoms in the primitive cell. More importantly, sound velocity and elastic properties unclosed the strong anharmonicity in In₄SnSe₄, which contributed greatly to the low thermal conductivity. With first-principles calculations, it was found that the lone-pair electrons from In⁺ mainly caused the anharmonicity in the lattice. Additionally, Br was proved to be an effective dopant for In₄SnSe₄ to improve the electrical transport properties. This work indicated that the complex wide-bandgap semiconductor In₄SnSe₄ with cubic phase and intrinsic low thermal conductivity was a new promising thermoelectric material with appropriate doping.

As a typical IV-VI compound, SnTe has aroused widely attentions in the thermoelectric community due its similar crystal and band structures with PbTe. However, both the large number of inherent Sn vacancies and high thermal conductivity result in inferior thermoelectric performance in intrinsic SnTe over a broad temperature. In this work, we successfully improved those disadvantages of SnTe via stepwisely Pb heavily alloying and then In doping. A significantly wide fraction of Pb into SnTe (0–50%) achieves multiple effects: (a) the carrier concentration of SnTe is reduced through decreasing Sn vacancies via alloying high solution Pb atoms in the matrix; (b) the band structure is optimized through promoting the convergence of the two valence bands, simultaneously enhancing the Seebeck coefficient; (c) HAADF-STEM coupled with EDS results illustrate that guest Pb atoms randomly and uniformly occupied Sn atomic sites in the matrix, concurrently strengthening the phonon scattering. Furthermore, we introduced indium into Sn_0.6Pb_0.4Te system to create resonant states further enlarging the power factors at low-medium temperature. The integration of bands convergence and DOS distortion achieves a considerably high ZT_ave of ~0.67 over the wide temperature range of 300–823 K in (Sn_0.6Pb_0.4)_0.995In_0.005Te sample.