Phase Formation Behavior and Thermoelectric Transport Properties of S-Doped FeSe_{2- x}S _x Polycrystalline Alloys

Some transition-metal dichalcogenides have been actively studied recently owing to their potential for use as thermoelectric materials due to their superior electronic transport properties. Iron-based chalcogenides, FeTe₂, FeSe₂ and FeS₂, are narrow bandgap (~1 eV) semiconductors that could be considered as cost-effective thermoelectric materials. Herein, the thermoelectric and electrical transport properties FeSe₂-FeS₂ system are investigated. A series of polycrystalline samples of the nominal composition of FeSe_2-xS_x (x = 0, 0.2, 0.4, 0.6, and 0.8) samples are synthesized by a conventional solid-state reaction. A single orthorhombic phase of FeSe₂ is successfully synthesized for x = 0, 0.2, and 0.4, while secondary phases (Fe₇S₈ or FeS₂) are identified as well for x = 0.6 and 0.8. The lattice parameters gradually decrease gradually with S content increase to x = 0.6, suggesting that S atoms are successfully substituted at the Se sites in the FeSe₂ orthorhombic crystal structure. The electrical conductivity increases gradually with the S content, whereas the positive Seebeck coefficient decreases gradually with the S content at 300 K. The maximum power factor of 0.55 mW/mK² at 600 K was seen for x = 0.2, which is a 10% increase compared to the pristine FeSe₂ sample. Interestingly, the total thermal conductivity at 300 K of 7.96 W/mK (x = 0) decreases gradually and significantly to 2.58 W/mK for x = 0.6 owing to the point-defect phonon scattering by the partial substitution of S atoms at the Se site. As a result, a maximum thermoelectric figure of merit of 0.079 is obtained for the FeSe_1.8S_0.2 (x = 0.2) sample at 600 K, which is 18% higher than that of the pristine FeSe₂ sample.