Investigation on phase formation and thermoelectric transport properties of CoTe₂-CoSe₂ solid solution alloys

CoTe₂ and CoSe₂ alloys have garnered considerable attention in thermoelectric applications due to their high electrical conductivity, tunable electronic properties, and potential for high power factors. In this study, the phase formation behavior and thermoelectric transport properties of a CoTe₂-CoSe₂ solid solution alloy were investigated by synthesizing a series of Co(Te_1−xSe_x)₂ compositions, where x = 0, 0.25, 0.50, 0.75, and 1.0. For x = 0, 0.25, and 0.50, the orthorhombic phase of CoTe₂ was observed, while for x = 0.75, the orthorhombic phase of CoTe₂ and cubic phase of CoSe₂ coexisted. Up to x = 0.50, the power factor decreased slightly as the Hall mobility and the density-of-state effective mass decreased. The total thermal conductivity decreased as x increased to 0.50, owing to a decrease in the electronic and lattice thermal conductivity. Despite the decrease in total thermal conductivity, no significant increase in zT values was observed in the range of x = 0.25-0.50 due to a more pronounced decrease in the power factor. Nevertheless, cubic phase CoSe₂ exhibited higher power factors and thus higher zT of 0.13 at 700 K. Further analysis based on single Kane band model suggests that significant enhancement in the power factor and zT could be achieved by reducing the carrier concentration from ∼10²¹ to ∼10¹⁹ cm⁻³ for all compositions.