Optimized thermoelectric transport properties in NiS₂–NiSe₂ system via solid solution alloying

Metal chalcogenide alloys are considered as electronic materials owing to their adjustable electrical properties either by doping or alloying. In this study, the electrical and thermal properties of the NiS₂–NiSe₂ system are investigated by synthesizing Ni(S_1-xSe_x)₂ (x = 0, 0.25, 0.5, 0.75, and 1.0) compositions. All samples exhibit pyrite-type cubic structures and form complete solid solution alloys. It is found that the NiS₂–NiSe₂ system exhibits a wide spectrum of electrical characteristics; NiS₂ exhibits semiconducting conduction with low electrical conductivity σ of 7.8 S/cm and a carrier concentration of ∼10¹⁹ cm⁻³ at room temperature, whereas NiSe₂ exhibits metallic conduction with high σ of 11,600 S/cm and ∼10²¹ cm⁻³ carriers. As x of Ni(S_1-xSe_x)₂ increased, the σ is gradually increased. On the other hand, the magnitude of Seebeck coefficient S is gradually decreased with x at 700 K. Thus, the optimized power factor (S²∙σ) of 0.10 mW/mK² at 700 K is achieved for Ni(S_0.75Se_0.25)₂ composition. Lattice thermal conductivities of the solid solution samples (x = 0.25, 0.5 and 0.75) range from 2.0 to 3.8 W/mK at 300 K, representing a reduction compared to those of NiS₂ and NiSe₂ (5.5 and 6.1 W/mK, respectively). Consequently, an enhanced thermoelectric performance was achieved in Ni(S_0.75Se_0.25)₂ benefiting from an optimized solid solution alloying, with a maximum thermoelectric figure of merit of 0.020 at 700 K.