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 α(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 κph of 0.32 W m−1 K−1 at ∼780 K is achieved along the [110] direction. Remarkably, this value is equivalent to the glassy limit κglass 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 κph 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.