Prediction of Fe₂TiSi Alloys as Efficient p-and-n-type Thermoelectrics at Low Temperatures via Explicit Treatment of Electron-phonon Scattering

Using first-principles methods based on DFT, we predict that alloys of Fe₂TiSi full-Heusler compound are potentially very efficient thermoelectric materials at low temperatures from 100 to 400 K. With about 4% of Ti replaced with Hf, p-type zT around 1 could be achieved, as well as slightly lower but still respectible n-type zT well beyond 0.5. p-and-n-type efficiencies are both high but for two different sets of physical reasons. High n-type power factor results from the well-known flat-and-dispersive conduction band. Even higher p-type power factor results from extremely high electron-phonon scattering lifetime at the valence band maximum and intrinsic Fermi level largely skewed towards the valence band. A large difference in effective masses of valence and conduction bands leads to the latter, allowing conductivity and Seebeck coefficient to be simultaneously high at low hole-doping concentrations. We explicitly calculate electron-phonon scattering matrix elements and lifetime by density functional perturbation theory and Wannier interpolation. Alloying reduces inherently high lattice thermal conductivity by mass-disorder scattering. If realized, these alloys of Fe₂TiSi would be very efficient low-temperature thermoelectrics as both p-and-n-types.