Phase segregation in high Mn-doped Mg₃SbBi thermoelectric materials

N-type Mg₃Sb₂-based thermoelectrics exhibit exceptionally high thermoelectric performance and superior mechanical stability. However, their performance is severely restricted by a high density of Mg vacancies and complex microstructure defects. In this work, we experimentally investigated the phase structure and thermoelectric performance of Mn-doped Mg₃SbBi compound. At low Mn doping level of 1%, Mn substitution in Mg sublattice boosted the electrical conductivity to 11.3 × S m^-1 and the power factor to 28.7 µw cm^-1 K^-2 at 300 K, which led to a remarkable enhancement in zT to 0.72 at room temperature. Meanwhile, the lattice thermal conductivity was reduced at temperature range of 350 to 650 K due to the acoustic phonon scattering leading zT to 1.3 at 420 K and further increases to 1.74 at 573 K. The average zT of 1.37 achieved which is highest achieved within 300-650 K temperature range. When Mn dopant became rich, high-resolution transmission electron microscopy and selected area electron diffraction revealed the existence of several resistive Mn-based phases (Mn₃Bi₂, Mn₂Sb, MnSb) at grain boundaries which acted as electron killers, degraded the thermoelectric performance, and switched the n-type behavior to p-type.