Exceptional thermoelectric properties in Na$_2$TlSb enabled by quasi-1D band structure

Abstract: Quasi-low-dimensional band structures in bulk high-symmetry materials can exhibiting density of states (DOS) profiles akin to those of low-dimensional materials. A particular striking example is the full-Heusler compound Na$2$TlSb, with valence band energy isosurfaces consisting of intersecting 2D-dimensional pockets forming a box-like structure, with the sides individually akin to the energy isosurfaces arising in one-dimensional (1D) quantum wells. The combination of high velocities and rapidly increasing DOS with energy is extremely promising for thermoelectric applications. However, these beneficial properties could in principle be counteracted by high electronic scattering rates, and the benefits of for instance band convergence has been questioned precisely because of increased scattering rates. It is therefore critical to understand the electronic scattering nature of low-dimensional systems. In the current study, describing electronic transport properties of Na$_2$TlSb from first principles, we found that electronic scattering rates remained modest despite the high DOS. This was caused by the intricate interplay between several effects: (i) delocalized energetic isosurfaces giving rise to large momentum scattering paths with low wavefunction overlap, reducing the scattering probability; (ii) a high DOS resulting in a high free-carrier screening; (iii) a large portion of the scattering paths within the flat energy isosurfaces keeps the electron group velocity nearly constant, reducing the effective relaxation rates. The high velocity and modest relaxation time result in a high carrier mobility, which, together with the high DOS and beneficial DOS profile, results in excellent electron transport properties. Combined with an ultra-low $\kappa\ell$ of $<1$ reported in literature, we predict a thermoelectric figure of merit ranging from 2.4 at 300 K to a maximum of 4.4 at 600 K.