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

Abstract

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(S1-xSex)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(S1-xSex)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(S0.75Se0.25)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(S0.75Se0.25)2 benefiting from an optimized solid solution alloying, with a maximum thermoelectric figure of merit of 0.020 at 700 K.