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

Abstract

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 (Fe1–xCox)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. (Fe1–xCox)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 (Fe0.5Co0.5)Te2 composition. The electronic band dispersions of FeTe2, (Fe0.5Co0.5)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 Fe0.5Co0.5Te2, supporting the high power factor of Fe0.5Co0.5Te2. 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 Fe0.5Co0.5Te2 compared to 0.001 for FeTe2 or 0.08 for CoTe2 at 600 K.