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Improving the figure of merit $zT$ of thermoelectric materials requiressimultaneously a high power factor and low thermal conductivity. An effectiveapproach for increasing the power factor is to align the band extremum andachieve high band degeneracy ($\geq$ 12) near the Fermi level as realized inPbTe [\textcolor{blue}{Pei et. al. \textit{Nature} 473, 66 (2010)}], whichusually relies on band structure engineering, e.g., chemical doping and strain.However, very few materials could achieve such a high band degeneracy withoutheavy doping or suffering impractical strain. By employing state-of-the-artfirst-principles methods with direct computation of phonon and carrierlifetime, we demonstrate that two new full-Heusler compounds Li$_2$TlBi andLi$_2$InBi, possessing a PbTe-like electronic structure, show exceptionallyhigh power factors ($\sim$ 20 mWm$^{-1}$K$^{-2}$ at 300 K) and low latticethermal conductivities (2.36 and 1.55 Wm$^{-1}$K$^{-1}$) at room temperature.The Tl$^{+}$Bi$^{3-}$ (In$^{+}$Bi$^{3-}$) sublattice forms a rock-saltstructure, and the additional two valence electrons from Li atoms essentiallymake these compounds isovalent with Pb$^{2+}$Te$^{2-}$. The larger rock-saltsublattice of TlBi (InBi) shifts the valence band maximum from L point to themiddle of the $\Sigma$ line, increasing the band degeneracy from fourfold totwelvefold. On the other hand, resonance bond in the PbTe-like sublattice andsoft Tl-Bi (In-Bi) bonding interaction is responsible for intrinsic low latticethermal conductivities. Our results present a novel strategy of designinghigh-performance thermoelectric materials.

Thermoelectric Alchemy: Designing A Chemical Analog to PbTe with Intrinsic High Band Degeneracy and Low Lattice Thermal Conductivity