High Thermoelectric Performance in SnTe-AgSbTe₂ Alloys from Lattice Softening, Giant Phonon-Vacancy Scattering, and Valence Band Convergence

We report on the underlying mechanism that enables the SnTe-AgSbTe₂ system to exhibit superior thermoelectric figure of merit (ZT) compared to its parent compound SnTe. We show that AgSbTe₂ alloying has a profound impact on the band structure of SnTe by converging the energies of its light and heavy valence bands, leading to significantly enhanced Seebeck coefficients. We have also unraveled a significant connection between alloying and defect stability in this system, wherein the Sn vacancy concentration increases significantly when Ag and Sb are alloyed on the Sn site. The increased Sn vacancy concentration dramatically reduces the lattice thermal conductivity through both lattice softening and phonon-vacancy scattering to ∼0.4 W m^-1 K^-1 at 800 K. Consequently, a ZT value of 1.2 at 800 K for AgSn₅SbTe₇ can be achieved by doping I on Te sites. This represents a 300% improvement over pristine SnTe, outperforming many reported SnTe-based thermoelectric materials.