Evolution of electrical transport properties in FeTe₂-CoTe₂ solid solution system for optimum thermoelectric performance

Transition metal chalcogenides have received significant attention as electronic materials owing to their tunable electronic transport properties and unique crystal structures. In this work, the electrical, thermal, and thermoelectrical transport properties of FeTe₂-CoTe₂ solid solution system were investigated by synthesizing a series of (Fe_1–xCo_x)Te₂ polycrystalline alloys with x = 0, 0.25, 0.5, 0.75, and 1. FeTe₂ and CoTe₂ exhibited identical orthorhombic structures and formed a complete solid solution. (Fe_1–xCo_x)Te₂ system exhibits wide range of electronic transport characteristics; CoTe₂ exhibits metallic conduction with high electrical conductivity of ∼8000 S/cm with electron carriers, whereas FeTe₂ exhibits semiconducting conduction with relatively low electrical conductivity of ∼300 S/cm with hole carriers at room temperature. The optimum carrier transport for high power factor of 1.53 mW/cmK² at 600 K is observed for (Fe_0.5Co_0.5)Te₂ composition. The electronic band dispersions of FeTe₂, (Fe_0.5Co_0.5)Te₂, and CoTe₂ were calculated by using the density functional theory and it was found that a distinct flat band is present near the Fermi level for Fe_0.5Co_0.5Te₂, supporting the high power factor of Fe_0.5Co_0.5Te₂. As the lattice thermal conductivity is reduced for the solid-solution samples with additional point defect scattering, the thermoelectric figure of merit (zT) increased significantly to 0.18 for Fe_0.5Co_0.5Te₂ compared to 0.001 for FeTe₂ or 0.08 for CoTe₂ at 600 K.