Enhanced Thermoelectric Properties of Fe1-xCoxSe2: Impact of Phase Transition and Point Defects

This study investigates the thermoelectric properties of Fe1-xCoxSe2 (x = 0, 0.25, 0.5, 0.75, and 1) alloys using theoretical modeling approaches. The Single Parabolic Band (SPB) model was employed to calculate band parameters such as density-of-states effective mass (md*), non-degenerate mobility (μ0), weighted mobility (μw), and B-factor. As the alloying content (x) increased, the B-factor improved, leading to higher predicted maximum zT values. For CoSe2 (x = 1), optimizing the carrier concentration to ~1.3 × 1019 cm-3 can potentially enhance zT to ~0.64, a 21-fold increase over the experimental value (~0.03). The Callaway-von Baeyer (CvB) model was used to analyze thermal transport properties, revealing that the increased x strengthens phonon scattering by point defects, resulting in a continuous decrease in lattice thermal conductivity (k1). Specifically, a phase transition from orthorhombic to cubic structure occurs as x increases, with a mixed phase observed at x = 0.75 and a complete transition to cubic at x = 1. This structural change significantly impacts both electronic and phonon band structures, leading to a dramatic increase in the B-factor and a substantial reduction in k1 for x > 0.75. These findings suggest that CoSe2 alloying, coupled with the induced phase transition, can significantly improve the thermoelectric performance of FeSe2-CoSe2 alloys.