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 (≥ 12) near the Fermi level as realized in PbTe [Pei et. al. 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 Li2TlBi and Li2InBi, possessing a PbTe-like electronic structure, show exceptionally high power factors (∼ 20 mWm-1K-2at 300 K) and low lattice thermal conductivities (2.36 and 1.55 Wm-1K-1) at room temperature. The Tl+Bi3-(In+Bi3-) sublattice forms a rock-salt structure, and the additional two valence electrons from Li atoms essentially make these compounds isovalent with Pb2+Te2-. The larger rock-salt sublattice of TlBi (InBi) shifts the valence band maximum from L point to the middle of the Σ 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.
|Original language||English (US)|
|State||Published - May 16 2018|
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