Thermoelectric Properties of a Light Compound FeS: the Role of Electron Correlation Strengthened Spin-Orbital Coupling.

Spin-orbit coupling (SOC) induced nontrivial bandgap and complex Fermi surface has been considered to be profitable for thermoelectrics, which, however, is generally appreciable only in heavy elements, thereby detrimental to practical application. In this study, the SOC-driven extraordinary thermoelectric performance in a light 2D material Fe₂S₂ is demonstrated via first-principles calculations. The abnormally strong SOC, induced by electron correlation through 3d orbitals polarization, significantly renormalizes the band structures, which opens the bandgap via Fe 3d orbitals inversion, exposes the second conduction valley with weak electron-phonon coupling, and aligns the energy of Fe 3d and S 3p orbitals with divergent momentum in valence band. Such topological band renormalization triggers improvement of both p- and n-type power factors by more than 200%. Combining with the low lattice thermal conductivity caused by lone pair electrons and intense high-order phonon scattering, the peak zT can reach 1.6 and 1.8 for p- and n-type Fe₂S₂ at 400 K, respectively. This work unravels the mechanism of SOC-provoked high zT in electron correlation systems, which inspires the development of high-performance thermoelectric materials without heavy and scarce elements.