Intermetallic compounds are usually poor thermoelectrics due to the high electronic densities of states at the Fermi level and concomitantly low Seebeck coefficients. However, intermetallic $B20$-type $\mathrm{Co}\mathrm{Si}$ has been experimentally shown to attain remarkably large negative values of the Seebeck coefficient. We provide a theoretical explanation for this surprising fact using state-of-the-art first-principles calculations with explicit treatment of electron-phonon interactions. We find that the unique band structure of $\mathrm{Co}\mathrm{Si}$, which has both massless and heavy fermion bands near the Fermi level, facilitates strong scattering of the low-energy electrons by phonons into the heavy band, resulting in effective energy filtering and high thermal voltage. Our study predicts that a very large thermoelectric power factor of 80 $\mu \mathrm{W}{\mathrm{cm}}^{-1}{\mathrm{K}}^{-2}$ or higher is experimentally accessible in the $300$–$600$ K range and highlights a general principle for identifying intermetallic compounds with large thermopower.

• Received 10 October 2018
• Revised 11 December 2018

DOI:https://doi.org/10.1103/PhysRevApplied.11.024017