Topological Electronic Transition Contributing to Improved Thermoelectric Performance in p-Type Mg3Sb2- xBix Solid Solutions

Abstract

Topological electronic transition is the very promising strategy for achieving high band degeneracy (N_V) and for optimizing thermoelectric performance. Herein, this work verifies in p-type Mg₃Sb_{2- x}Bi_x that topological electronic transition could be the key mechanism responsible for elevating the N_V of valence band edge from 1 to 6, leading to much improved thermoelectric performance. Through comprehensive spectroscopy characterizations and theoretical calculations of electronic structures, the topological electronic transition from trivial semiconductor is unambiguously demonstrated to topological semimetal of Mg₃Sb_{2- x}Bi_x with increasing the Bi content, due to the strong spin-orbit coupling of Bi and the band inversion. The distinct evolution of Fermi surface configuration and the multivalley valence band edge with N_V of 6 are discovered in the Bi-rich compositions, while a peculiar two-step band inversion is revealed for the first time in the end compound Mg₃Bi₂. As a result, the optimal p-type Mg₃Sb_0.5Bi_1.5 simultaneously obtains a positive bandgap and high N_V of 6, and thus acquires the largest thermoelectric power factor of 3.54 and 6.93 µW cm^-1 K^-2 at 300 and 575 K, respectively, outperforming the values in other compositions. This work provides important guidance on improving thermoelectric performance of p-type Mg₃Sb_{2- x}Bi_x utilizing the topological electronic transition.

Keywords: Mg3Sb2−xBix; multivalley valence band; thermoelectric; topological electronic transition.