Effect of Bi-substitution on structural stability and improved thermoelectric performance of p-type half-Heusler TaSbRu: A first-principles study

Abstract

Recently, Fang et al. have predicted a high thermoelectric (TE) figure of merit (ZT) of 1.54 in TaSbRu alloys at 1200 K from first-principles calculations without considering spin-orbit interaction, accurate electronic structure, details of phonon scattering, and energy-dependent holes relaxation time. Here, we report the details of structural stability and thermoelectric performance of Bi-substituted p-type TaSbRu from first-principles calculations considering these important parameters. This indirect bandgap semiconductor (Eg = 0.8 eV by Tran-Blaha modified Becke Jonshon potential (TB-mBJ) including spin-orbit coupling (SOC) effect) has highly dispersive and degenerate valence bands, which lead to a maximum power factor, 3.8 mW m−1 K−2 at 300 K. As Sb-5p has a small contribution to the bandgap formation, the substitution of Bi on the Sb site does not cause significant change to the electronic structure. Although the Seebeck coefficient increases by Bi due to slight changes in the bandgap, electrical conductivity decreases, and hence, the power factor reduces to ~3 mW m−1 K−2 at 300 K (50% Bi). On the other side, lattice thermal conductivity drops effectively to 5 from 20 W m−1 K−1 as Bi introduces a significant contribution to the acoustic phonons and intensifies phonon scattering. Thus, ZT value is improved through Bi-substitution, reaching 1.1 (50% Bi) at 1200 K from 0.45 (pure TaSbRu) only. Therefore, the present study suggests how to improve the TE performance of Sb-based half-Heusler compounds and TaSbRu (with 50% Bi) is a promising material for high-temperature TE applications.