Significant Enhancement in the Thermoelectric Performance of the MnSb2Te4 Topological Insulator through Vacancy Regulation and Lattice-Softening Strategies

The topological insulator MnSb2Te4 shows promising potential in thermoelectric applications due to its intrinsically low lattice thermal conductivity. However, its thermoelectric performance is limited by the high carrier concentration, of which the origin is still unclear. In this work, the carrier concentration is successfully tuned from 2.24 × 1021 cm-3 to as low as 9.1 × 1019 cm-3. Transmission electron microscopy and positron annihilation measurements suggest that large amounts of Mn vacancies exist in the septuple layer of MnSb2Te4, which are responsible for the high carrier concentration. The Mn vacancies are suppressed by the excess Mn element and AgSbTe2 alloying, which not only reduces the carrier concentration but also weakens the carrier scattering and thus improves the mobility. The decrease in carrier concentration also leads to reduced electronic thermal conductivity. The excess Mn atoms introduce a strain field in the Mn layer, which enhances phonon scattering. Furthermore, the substitution of Ag for Mn causes lattice softening by weakening the chemical bonds in MnSb2Te4, which leads to reduced phonon velocity and, therefore, further reduction in lattice thermal conductivity. As a result, an extremely low lattice thermal conductivity of 0.44 W m-1 K-1 was obtained at 300 K and it further decreased to 0.17 W m-1 K-1 at 798 K. Finally, a record zT value of 1.53 at 798 K was achieved in Mn1.06Sb2Te4(AgSbTe2)0.04, and the optimal carrier concentration is about 2 × 1020 cm-3.