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

Abstract

Improving the figure of merit $zT$ of thermoelectric materials requires simultaneously a high power factor and low thermal conductivity. An effective approach for increasing the power factor is to align the band extremum and achieve high band degeneracy ($\ge$ 12) near the Fermi level as realized in PbTe [\textcolor{blue}{Pei et. al. \textit{Nature} 473, 66 (2010)}], which usually relies on band structure engineering, e.g., chemical doping and strain. However, very few materials could achieve such a high band degeneracy without heavy doping or suffering impractical strain. By employing state-of-the-art first-principles methods with direct computation of phonon and carrier lifetime, we demonstrate that two new full-Heusler compounds Li${}_2$TlBi and Li${}_2$InBi, possessing a PbTe-like electronic structure, show exceptionally high power factors ($\sim$ 20 mWm${}^{-1}$K${}^{-2}$ at 300 K) and low lattice thermal conductivities (2.36 and 1.55 Wm${}^{-1}$K${}^{-1}$) at room temperature. The Tl${}^{+}$Bi${}^{3-}$ (In${}^{+}$Bi${}^{3-}$) sublattice forms a rock-salt structure, and the additional two valence electrons from Li atoms essentially make these compounds isovalent with Pb${}^{2+}$Te${}^{2-}$. The larger rock-salt sublattice of TlBi (InBi) shifts the valence band maximum from L point to the middle of the $\Sigma$ line, increasing the band degeneracy from fourfold to twelvefold. On the other hand, resonance bond in the PbTe-like sublattice and soft Tl-Bi (In-Bi) bonding interaction is responsible for intrinsic low lattice thermal conductivities. Our results present a novel strategy of designing high-performance thermoelectric materials.

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