  Recently, Fang et al. have predicted a high ZT of 1.54 in TaSbRu alloys at
1200 K from first-principles 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 theses important
parameters. This indirect bandgap semiconductor (Eg=0.8 eV by TB-mBJ+SOC) has
highly dispersive and degenerate valence bands, which lead to a maximum power
factor, 3.8 mWm-1K-2 at 300K. 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, and hence,
the power factor reduces to ~3 mW m-1K-2 at 300K (50% Bi). On the other side,
lattice thermal conductivity drops effectively to 5 from 20 W/m K as Bi
introduces a significant contribution in the acoustic phonon region and
intensify 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 applications.
