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. 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.