Strong influence of fully quartic anharmonicity on thermoelectric properties of Li3X (X= Sb, Bi) compounds: A first principle study

Abstract

In this work, the thermal transport and thermoelectric properties of Li₃X (X = Sb, Bi) have been systematically investigated by utilizing first-principles calculations combined with self-consistent phonon theory and Boltzmann transport equations. The fully quartic anharmonicity of Li₃X is considered in the lattice thermal conductivity (κ_L) calculation, which encompasses temperature-driven phonon frequency shift and four-phonon (4ph) scattering. Our results indicate that fully quartic anharmonicity plays a crucial role in accurately determining the κ_L and the figure of merit (ZT). The acoustic phonon group velocity (υ_ph) is enhanced due to the quartic anharmonic renormalization. The three-phonon (3ph) scattering is suppressed and an additional 4ph scattering can be introduced by self-consistent phonon theory combined with 3ph and 4ph scatterings. The phonon lifetime (τ) ultimately exhibits negligible variation. The relatively rational κ_L value of 1.18 and 0.94 W/(mK) are obtained at 900 K for Li₃Sb and Li₃Bi crystals, respectively. Meanwhile, the coexistence of the large dispersion and high degeneracy in the electronic structure capture a big power factor. Thus, we obtain high ZT peak values of 1.56 and 2.25 for the p-type Li₃Sb and Li₃Bi materials at 900 K, respectively. The superior value 2.25 exceeding 2 indicates the great potential of p-type Li₃Bi in thermoelectric applications. These findings help us to well understand the microcosmic mechanism of thermoelectric properties in Li₃X, and give useful insights into the role of the fully quartic anharmonicity for thermoelectric properties of Li₃X.