Enhanced Thermoelectric Properties of FeSe₂ Alloys by Lattice Thermal Conductivity Reduction by Cl Doping

Metal chalcogenides are widely studied as thermoelectric materials due to their finely tunable electronic transport properties over a wide temperature range. FeSe₂ has recently been considered a promising thermoelectric material with investigations focusing on restraining bipolar behavior through doping. In this study, a series of Cl-doped FeSe₂ compositions, a series of FeSe_{2 − x}Cl_x (x = 0, 0.01, 0.025, and 0.05) compositions, were synthesized to investigate the influence of Cl doping. While the gradually decreasing lattice parameters with doping content x suggests successful doping up to x = 0.05, the hole concentration slightly decreased owing to electrons generated by the Cl doping. Nevertheless, the electrical conductivity and Seebeck coefficient show no systematic change with x owing to very low electron generating efficiency, and no distinctive enhancement of power factor is seen for the doped samples. On the other hand, the lattice thermal conductivity gradually and significantly decreased with x from 9.2 W/mK to 6.3 W/mK for x = 0.05 by 32% at 300 K, which is originated from the effective additional phonon scattering due to the difference in mass (55%) and size (9%) between Se²⁻ and Cl⁻ ions. Consequently, a thermoelectric figure of merit is increased to 0.073 from 0.057 at 600 K for x = 0.05.