Exploring high-performance thermoelectric materials with improved mechanical properties is important for broadening the application scope and the assembly requirement of stable devices. This work presents an effective strategy to discover hard thermoelectric material by inserting foreign atoms in the rigid covalent framework. We demonstrate this in boron-carbon clathrate Ⅶ structure, showing a promising candidate for highly efficient thermoelectric energy conversion, especially with Y atom filled in the cage, with a peak zT of 0.73 at 1, 000 K. The ab initio calculations indicate that YB₃C₃ system has low lattice thermal conductivity of 4.5 W/(m·K) at 1, 000 K due to the strong rattling of encaged Y atom. The strongly covalent framework provides highly degenerate band structures consisting of heavy and light electron pockets, which can maintain high carrier mobility arising from small effective mass and thus large group velocity. Consequently, high power factor can be achieved in YB₃C₃ for both electron and hole doping. In addition, it exhibits well mechanical properties and a Vickers hardness of 23.7 GPa because of the strong covalent boron-carbon framework. This work provides a novel avenue for the search of high-performance thermoelectric materials with excellent mechanical properties, based on boron-carbon clathrate structure.

Transition-metal sulfides, such as 1T- and 2H-TaS₂, are attracting considerable interest in modern condensed matter physics for their diverse behaviors of the Mott state, peculiar charge-density-wave phase and superconductivity. The intrinsically low thermal conductivities along the cross-plane direction can advantage the potential high thermoelectric performance; yet, their insignificant power factors severely hampered the practical applications as thermoelectric devices. In this perspective, we herein present a new semiconducting phase in TaS₃ with the space group C2/m predicted by the swarm-intelligence structure-searching method. The C2/m-TaS₃ phase exhibits anisotropic multivalley band dispersions, which is beneficial for electronic transport. Meanwhile, the unique structure within nanopores leads to strong anharmonic scattering, significantly reducing the lattice thermal conductivity. As a result, the calculated figure of merit ZT can reach up to 1.68 and 1.57 at 800 K for p- and n-type, respectively that is comparable with conventional thermoelectric materials (e.g. PbTe, Bi₂Te₃). Therefore, our calculation reveals that the C2/m-TaS₃ phase can be a potential high-performance candidate as non-toxic and eco-friendly thermoelectrics, and will stimulate further experimental exploration for understanding and tailoring thermoelectric capability in related transition-metal sulfides.