Evolution of electrical, thermal, and thermoelectric transport properties of solid-solution alloys of CoSe₂ and CoS₂

CoSe₂ and CoS₂, possessing the same pyrite-type crystal structures, are semiconducting materials which exhibit semi-metallic behavior, which can be considered as promising materials in the fields of thermoelectricity and energy storage. In this study, the electrical and thermal transport properties of complete solid-solution alloys between CoSe₂ and CoS₂ (Co(Se_1-xS_x)₂, x = 0, 0.25, 0.5, 0.75, and 1.0) are investigated. It was first confirmed that all the compositions form a single phase of complete solid solutions by X-ray diffraction and energy-dispersive spectroscopy. As forming the complete solid solution with x increases, carrier concentration and mobility gradually decreased as x increases to 0.75, leading to a corresponding reduction in the power factor. For instance, the power factor decreases by 45% from 1.4 mW/mK² for CoSe₂ to 0.77 mW/mK² for x = 0.75 at 700 K. The reduced lattice thermal conductivities of 2.10, 1.67, and 2.28 W/mK were measured at 700 K for the solid-solution alloy compositions of x = 0.25, 0.5, and 0.75, respectively, owing to additional point-defect scattering that originated from solid-solution alloying. It can be noted that the reduction in lattice thermal conductivity was maximized at x = 0.5 where the additional scattering is expected to be maximized. Nevertheless, the thermoelectric figure of merit (zT) of the Co(Se_1-xS_x)₂ solid-solution sample (x = 0.25, 0.5, and 0.75) is decreased to 0.087–0.095 compared to those of CoSe₂ and CoS₂ (0.13 and 0.097, respectively) at 700 K due to rather large reduction of power factors. Furthermore, further analysis on the electrical and thermal transport properties based on single parabolic band model was performed and found that the zT of the x = 0.5 can be further enhanced to outrage the pristine samples by reducing carrier concentration owing to the maximum lattice thermal conductivity reduction, despite of the power factor degradation.