High Thermoelectric Performance and Defect Energetics of Multipocketed Full Heusler Compounds
We report a first-principles density-functional study of electron-phonon interactions in and thermoelectric transport properties of the full Heusler compounds Sr2BiAu and Sr2SbAu. Our results show that ultrahigh intrinsic bulk thermoelectric performance across a wide range of temperatures is physically possible and point to the presence of multiply degenerate and highly dispersive carrier pockets as the key factor for achieving this. Sr2BiAu, which features ten energy-aligned low-effective-mass pockets (six along Γ−X and four at L), is predicted to deliver n-type zT=0.4–4.9 at T=100–700 K. Comparison with the previously investigated compound Ba2BiAu shows that the additional L pockets in Sr2BiAu significantly increase its low-temperature power factor to a maximum value of 12 mW m−1 K−2 near T=300 K. However, at high temperatures the power factor of Sr2BiAu drops below that of Ba2BiAu because the L states are heavier and subject to strong scattering by phonon deformation, as opposed to the lighter Γ−X states, which are limited by polar-optical scattering. Sr2SbAu is predicted to deliver a lower n-type zT=3.4 at T=750 K due to appreciable misalignment between the L and Γ−X carrier pockets, generally heavier scattering, and a slightly higher lattice thermal conductivity. Soft acoustic modes, which are responsible for the low lattice thermal conductivity, also increase the vibrational entropy and high-temperature stability of these Heusler compounds, suggesting that their experimental synthesis may be feasible. The dominant intrinsic defects are found to be Au vacancies, which drive the Fermi level towards the conduction band and work in favor of n-doping.