Mechanisms behind the weighted mobility improvement in FeTe₂-CoTe₂ thermoelectric solid solution alloys

Fe-based dichalcogenides, captivating interest for their narrow 3d bands and potential as thermoelectric materials, have recently garnered attention. Park et al. reported enhanced zT of 0.18 for (Fe_1-xCo_x)Te₂ (x = 0.5) at 600 K, attributing it to the emergence of a flat band near the Fermi level. However, the influence of alloying-induced band engineering and point defect-mediated phonon scattering remains elusive. Herein, we investigate the impact of compositional variation (x) on electronic transport properties using the Single Kane Band model. Our analysis reveals predominant band convergence (x < 0.5) transitioning to band sharpening (x > 0.5). Notably, the weighted mobility, directly linked to the theoretical maximum power factor, exhibits a pronounced increase with increasing x. To evaluate the strength of phonon scattering by point defects, the Callaway-von Baeyer (CvB) model was employed. Surprisingly, the highest scattering parameter was not observed at x = 0.5 (maximum mixing) but at x = 0.25 and 0.75, signifying stronger phonon scattering at these compositions. Finally, considering the weighted mobility and lattice thermal conductivity, the theoretical maximum zT was calculated. Our findings predict a potential enhancement of 118 times in zT for x = 0.75 (0.68) compared to the experimental value at x = 0 (0.005). This study underscores the significant potential for discovering novel, unexplored metal chalcogenide-based solid solutions with enhanced thermoelectric performance.