Utilizing Band Anisotropy to Enhance the Thermoelectric Power Factor

The electronic band structure is a “gold mine” for exploring novel strategies that will promote the discovery of good thermoelectrics. Band anisotropy is a key characteristic of electronic structure that broadly exists in a lot of solid materials. Theoretically, valley anisotropy was proposed to benefit the electrical transport of TE materials [1,2]. However, there is still no experimental demonstration to confirm this prediction due to the challenge of the selection of suitable material as a model. Here, taking advantage of the single anisotropic Fermi pocket in p-type Mg3Sb2, a feasible strategy of utilizing the valley anisotropy to enhance the thermoelectric properties was demonstrated by synergistic studies of single crystals and textured polycrystalline samples. Compared to the heavy-band direction, improved carrier mobility by a factor of 3 is observed along the light-band direction while the Seebeck coefficient remains similar. Together with lower lattice thermal conductivity, increased room-temperature zT by a factor of 3.6 was found in the light-band direction of the single crystal. Further analysis revealed that the pz orbital, which overlaps more along the light-band direction, leads to the anisotropic valence band. With this knowledge, first-principles calculations of 66 isostructural Zintl phase compounds were conducted and 9 of them were found to display a pz orbital-dominated valence band, which are useful for guiding the development of high-performance p-type Zintl phase TE materials [3].<br/><br/>[1] Phy. Rev. Lett. 110, 146601 (2013)<br/>[2] Phys. Rev. Lett. 114, 136601 (2015)<br/>[3] Nat. Commun. 12, 5408 (2021)