Phase formation behavior and electronic transport properties of HfSe2-HfTe2 solid solution system

Abstract

Transition metal dichalcogenides have gained renewed attention as promising thermoelectric materials primarily because of their unique structures and superior electronic properties. In this study, we investigate the electrical transport properties of a series of Hf(Se,Te)2 samples (Hf(SexTe1-x)2, x = 0, 0.025, 0.25, 0.375, 0.5, and 1). All the samples form a single phase of a complete solid solution, displaying a wide variety of electrical properties based on the evolution of the composition. HfTe2 exhibits metallic conduction with very high electrical conductivity of ~1600 S/cm, whereas HfSe2 exhibits semiconducting conduction with very low electrical conductivity of 0.33 S/cm at room temperature. In contrast, the Seebeck coefficient of the HfTe2 sample is as low as 17 μV/K, whereas that of HfSe2 approaches 730 μV/K. Therefore, the power factor for HfTe2 represents a much higher value of 0.24 mW/mK2 when compared with 0.01 mW/mK2 for HfSe2 at 600 K. The superior electrical transport properties of HfTe2 are primarily caused by its high carrier concentration of 1021 cm−3 and high density-of-state effective mass of 0.91m0. A comparison of the electronic band dispersion between HfSe2 and HfTe2 calculated using the density functional theory indicates that the dispersions of the conduction and valence bands seem to remain unchanged: HfSe2 exhibits a finite band gap, whereas HfTe2 exhibits no or a very small band gap.