The Importance of Mg–Sb Interactions in Achieving High Conduction Band Degeneracy in Mg3Sb2 for High n-Type Thermoelectric Performance

Abstract

In recent years, Mg3Sb2 has attracted much attention due to its excellent performance as an n-type thermoelectric material. The high performance of n-type Mg3Sb2 is largely due to the high valley degeneracy (NV) of the electron transport edge, which is the result of the conduction band minimum (CBM) being located at a low-symmetry, six-fold degenerate point (inside the ΓALM plane) in the first Brillouin zone. Furthermore, Bi alloying on the Sb site is well-known to improve electron mobility, and hence, performance. In previous works, this low-symmetry CBM has been attributed to interactions between the tetrahderally and octahedrally coordinated Mg-sites. However, these Mg–Mg interactions cannot explain changes in the shape of the CBM upon Bi alloying, while Mg–Sb (and Mg–Bi) interactions could explain them. Prior studies have neglected to fully consider the importance of the Sb orbitals in the existence of the low-symmetry CBM because at the CBM, there is very little contribution from the Sb orbitals. In this work, we use first-principles density functional theory and tight-binding calculations to show that the low-symmetry CBM is due to Mg-s–Sb-p interactions that are weakest at or near the CBM. Controlling the cation-s–anion-p interactions through anion- or cation-site alloying can be used as an engineering strategy to optimize thermoelectric performance.