Charge and thermal transport properties of undoped and doped T l + ( I n 3 + X 2 2 − ) − ternary dichalcogenides with a reduced dimensionality: perfect candidates for thermoelectric applications in the mid-temperature region

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 T l + ( I n 3 + X 2 2 − ) − 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 T l + ( I n 3 + X 2 2 − ) − samples over a wide temperature range between ∼100 K and ∼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.