Fermi surface complexity, effective mass, and conduction band alignment in n-type thermoelectric Mg3Sb2 – xBix from first principles calculations

Using first principles calculations, we study the conduction band alignment, effective mass, and Fermi surface complexity factor of n-type Mg 3Sb 2 - xBi x (x = 0, 1, and 2) from the full ab initio band structure. We find that with an increase in the Bi content, the K and M band minima move away from the conduction band minimum CB 1 while the singly-degenerate Cyrillic capital letter GHE band minimum shifts rapidly downward and approaches the conduction band minimum. However, the favorable sixfold degenerate CB 1 band minimum keeps dominating the conduction band minimum and there is no band crossing between the Cyrillic capital letter GHE and CB 1 band minima. In addition, we show that the connection of the CB 1 carrier pockets with the energy level close to the band minimum M can strongly enhance the carrier pocket anisotropy and Fermi surface complexity factor, which is likely the electronic origin for the local maximum in the theoretical power factor. Our calculations also show that the density of states effective mass, Seebeck coefficient, and Fermi surface complexity factor decrease with an increase in the Bi content, which is unfavorable to the electrical transport. In contrast, reducing the conductivity effective mass with an increase in the Bi content is beneficial to the electrical transport by improving carrier mobility and weighted mobility as long as the detrimental bipolar effect is insignificant. As a result, in comparison with n-type Mg 3Sb 2, n-type Mg 3SbBi shows higher power factors and a much lower optimal carrier concentration for the theoretical power factor at 300 K, which can be easily achieved by the experiment.