Doubled Thermoelectric Figure of Merit in p-Type β-FeSi₂ via Synergistically Optimizing Electrical and Thermal Transports

β-FeSi2 has long been investigated as a promising thermoelectric (TE) material working at high temperatures due to its combining features of environmental friendliness, good thermal stability, and strong oxidation resistance. However, the real application of β-FeSi2 is still limited by its low TE figure of merit (zT). In this study, nearly doubled zT in p-type β-FeSi2 has been achieved via synergistically optimizing electrical and thermal transports. Based on the first-principles calculations, Al with shallow acceptor transition level and high carrier donation efficiency is chosen to dope β-FeSi2. Significantly improved electrical transport, particularly in the low temperature range, has been obtained in the Al-doped β-FeSi2 system. The power factor for FeSi1.96Al0.04 at 300 K is even higher than that of p-type β-FeSi2-based compounds reported previously at high temperatures. By alloying β-FeSi2 with Os at the Fe sites, we further lower the lattice thermal conductivity. Fe0.80Os0.20Si1.96Al0.04 possesses the lowest lattice thermal conductivity among the β-FeSi2 compounds prepared by the equilibrium method. Finally, a record-high zT value of 0.35 is obtained for p-type Fe0.80Os0.20Si1.96Al0.04. This study is expected to accelerate the application of β-FeSi2.