Synthesis and physical properties of single-crystalline InTe: towards high thermoelectric performance

Chalcogenide semiconductors and semimetals continue to be of prime interest for thermoelectric applications in power generation. As another representative of this broad class of materials, tetragonal InTe has recently emerged as a promising candidate due to its very limited ability to transport heat leading to high thermoelectric performance near 800 K in polycrystalline samples. However, little is known on the basic physical mechanisms governing its electronic and thermal properties, an in-depth study of which requires the growth of single crystals. Here, we report a detailed investigation of the transport properties of InTe single crystals grown by the Bridgman-Stockbarger technique over a wide range of temperatures (5-800 K). Except for the Hall coefficient that remains nearly isotropic below 300 K, all the transport coefficients show a significant anisotropy between the c and [110] direction of the crystal structure. In contrast to electronic band structure calculations suggesting a semimetallic ground state, the high-temperature dependence of the thermopower alpha(T) indicates that InTe is a semiconductor with a band gap estimated to be 0.26 eV from the Goldsmid-Sharp relation. Despite the absence of grain boundary scattering, an extremely low lattice thermal conductivity kappa(ph) of 0.32 W m(-1) K-1 at similar to 780 K is achieved along the [110] direction. Remarkably, this value is equivalent to the glassy limit kappa(glass) based on phonon-mediated heat transport suggesting that, at high temperatures, the thermal transport in InTe has reached its minimum value. The combination of extremely low kappa(ph) values with a relatively high power factor yields a maximum dimensionless thermoelectric figure of merit ZT of 0.61 at 780 K along the [110] direction. The present study provides a solid basis for future doping strategies of InTe for high-temperature thermoelectric applications.

Automated summary

Chalcogenide semiconductors and semimetals, including tetragonal InTe, are promising candidates for thermoelectric applications due to their high thermoelectric performance near 800 K. An investigation of InTe single crystals grown over a wide temperature range revealed anisotropic transport coefficients and a semiconductor ground state despite previous suggestions of a semimetallic nature. The extremely low lattice thermal conductivity of InTe, combined with a relatively high power factor, resulted in a maximum dimensionless thermoelectric figure of merit of 0.61 at 780 K along the [110] direction, laying a solid foundation for future doping strategies in high-temperature thermoelectric applications.