Topological Electronic Transition Contributing to Improved Thermoelectric Performance in p-Type Mg 3 Sb 2- x Bi x Solid Solutions.

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 Mg 3 Sb 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 Mg 3 Sb 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 Mg 3 Bi 2 . As a result, the optimal p-type Mg 3 Sb 0.5 Bi 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 Mg 3 Sb 2- x Bi x utilizing the topological electronic transition.