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 <i>zT</i> value of 1.53 at 798 K was achieved in Mn_1.06Sb₂Te₄(AgSbTe₂)_0.04, and the optimal carrier concentration is about 2 × 10²⁰ cm^-3.