Enhanced thermoelectric properties of YbZn₂Sb_2−xBi_x through a synergistic effect via Bi-doping

Recently, extensive research has been done to explore the potential thermoelectric performance of p-Type Zintl compounds AB₂Sb₂ (A = Ca, Yb, Eu, Sr, Mg; B = Zn, Mn, Cd, Mg), mainly focus on A-site substitution and/or B-site substitution effect. Herein, we report that both power factors and thermal conductivities are simultaneously optimized to increase ZT values via substitution of Sb atoms with the isoelectronic Bi atoms in YbZn₂Sb_2−xBi_x compounds. We found that the power factor could be enhanced by 27%, as a result of improved electrical conductivities and the maintained Seebeck coefficients. The former is obtained from an increase of carrier density through a shift of Fermi level via Bi doping, which is supported by XPS analysis. The Seebeck coefficients are maintained due to the increased effective mass. Moreover, the lowest lattice thermal conductivity (0.47 Wm⁻¹ K⁻¹ at 723K) is below the lattice thermal conductivity limit due to enhanced phonon scattering via Bi-doping, which is analyzed by the Callaway model. Benefiting from the synergistic effect, the ZT of Bi-doping samples are significantly enhanced compared with that of the pristine one. These results probably offer a synergistic strategy to enhance ZT for p-Type Zintl compounds AB₂Sb₂ and other high-efficiency thermoelectric materials.