One-dimensional van der Waals BiSBr: an anisotropic thermoelectric mineral.

The fascinating characteristics of one-dimensional van der Waals crystals (V-VI-VII) enable their wide functionality. In particular, their anisotropic carrier transport and low thermal conductivity are advantageous from a thermoelectric viewpoint. In a quest for the "electron crystal phonon glass" paradigm, the present work investigated the thermoelectric performance of BiSBr. Deep insights were gained into the structural and electronic properties that revealed the synergetic effect of the bonding heterogeneity, lone pair of electrons, and degenerated bands that accelerated favorable transportation. Consequently, a low lattice thermal conductivity (0.225 W m-1 K-1) and considerable power factor (3.471 mW m-1 K-2) and thus a high zT of 2.34 were noted in the x-direction (perpendicular to the chain) at 500 K in the case of the material with 1.5 × 1020 holes per cm3. These observations suggest that BiSBr is a plausible near-room-temperature anisotropic thermoelectric mineral.

Pressure-driven thermoelectric properties of defect chalcopyrite structured ZnGa2Te4: ab initio study.

The pressure induced structural, electronic, transport, and lattice dynamical properties of ZnGa2Te4 were investigated with the combination of density functional theory, Boltzmann transport theory and a modified Debye-Callaway model. The structural transition from I4̄ to I4̄2m occurs at 12.09 GPa. From the basic observations, ZnGa2Te4 is found to be mechanically as well as thermodynamically stable and ductile up to 12 GPa. The direct band gap of 1.01 eV is inferred from the electronic band structure. The quantitative analysis of electron transport properties shows that ZnGa2Te4 has moderate Seebeck coefficient and electrical conductivity under high pressure, which resulted in a large power factor of 0.63 mW m-1 K-2 (750 K). The ultralow lattice thermal conductivity (∼1 W m-1 K-1 at 12 GPa) is attributed to the overlapping of acoustic and optical phonon branches. As a result, the optimal figure of merit of 0.77 (750 K) is achieved by applying a pressure of 12 GPa. These findings support that ZnGa2Te4 can be a potential p-type thermoelectric material under high pressure and thus open the door for its experimental exploration.