Charge and thermal transport properties of undoped and doped Tl+(In3+X22-)- ternary dichalcogenides with a reduced dimensionality: perfect candidates for thermoelectric applications in the mid-temperature region

Abstract

The present work focuses on a comprehensive investigation of charge and thermal transport properties of thallium-indium-based ternary dichalcogenides with a common chemical formula of Tl+(In3+X22-)- where X denotes tellurium, selenium or sulfur atoms. Two compounds in the one-dimensional chain form, including pristine TlInSe2 and Fe-doped TlInTe2 as well as a hybrid material constructed from the two-dimensional TlInS2 layered semiconductor diluted with TlFeS2 at % 0.7, have been successfully grown using the Bridgman-Stockbarger technique. X-ray powder diffraction, scanning electron microscopy and energy-dispersive x-ray spectroscopy measurements were performed to characterize the local structure and chemical composition features of all prepared compounds. A strong anisotropy in the charge-carrier transport properties of the samples was observed from dc- and ac- electrical conductivity measurements made in directions parallel and perpendicular to the layers/chains. This is a fundamental property for realizing high thermoelectric performance in semiconducting materials. As a result, extremely large Seebeck coefficients were revealed upon experimental investigations of the thermoelectric properties of Tl+(In3+X22-)- samples over a wide temperature range between similar to 100 K and similar to 800 K. The first-principles density functional theory (DFT) and Boltzmann transport equations were employed to investigate the thermal transport properties of the compounds studied at the atomic scale. A specific interaction between the thallium cation and the sulfur anion was observed in the DFT computation scheme. The developed interaction (more electrostatic than a much weaker van der Waals one) enhances the charge carrier effective mass via flattening of the electronic bands near the band edges and this can give a new path towards Seebeck coefficient enhancement in the traditional mid-temperature range.