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

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(Se _xTe _1-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. HfTe ₂ exhibits metallic conduction with very high electrical conductivity of ~1600 S/cm, whereas HfSe ₂ exhibits semiconducting conduction with very low electrical conductivity of 0.33 S/cm at room temperature. In contrast, the Seebeck coefficient of the HfTe ₂ sample is as low as 17 μV/K, whereas that of HfSe ₂ approaches 730 μV/K. Therefore, the power factor for HfTe ₂ represents a much higher value of 0.24 mW/mK ² when compared with 0.01 mW/mK ² for HfSe ₂ at 600 K. The superior electrical transport properties of HfTe ₂ are primarily caused by its high carrier concentration of 10 ²¹ cm ⁻³ and high density-of-state effective mass of 0.91 m ₀. A comparison of the electronic band dispersion between HfSe ₂ and HfTe ₂ calculated using the density functional theory indicates that the dispersions of the conduction and valence bands seem to remain unchanged: HfSe ₂ exhibits a finite band gap, whereas HfTe ₂ exhibits no or a very small band gap.