Band convergence is a traditional strategy to optimize the electrical transport properties of SnTe, but conventionally optimized band structures often retain the valence band maximum (VBM) at the low-degeneracy L point $(N\mathrm{v}=4)$. Moreover, the mechanisms of phonon softening remain poorly understood in terms of phonon dispersion, especially regarding the coupling between vacancies and other dopants. Here, the strategic incorporation of ternary ${\mathrm{MnSb}}_2{\mathrm{Te}}_4$ into SnTe via nonstoichiometric alloying introduces a synergistic effect of Sn vacancies and Mn dopants. This synergy weakens the Sn-s/Te-p antibonding state, leading to significant band renormalization. Specifically, the VBM shifts from the L point to the high-degeneracy Σ and Λ points ($N\mathrm{v}=12$ and 8, respectively), where the VBM at these two sites form perfect alignment, substantially enhancing the density-of-states effective mass. In terms of phonon dispersion, both Mn atoms and Sn vacancies soften optical phonon modes at the Γ point, reducing their frequencies from 0.9 THz to 0.3 and 0.6 THz, respectively, forming a pronounced optical-phonon “waterfall” effect. This results in a significant reduction in phonon group velocities and introduces additional optical-acoustic phonon interactions. These combined effects yield a highly optimized thermoelectric quality factor in SnTe-${\mathrm{MnSb}}_2{\mathrm{Te}}_4$ alloys. By further tuning the reduced Fermi level through Ge doping, we achieve a maximum ZT of 1.3 at 723 K and an average ZT of 0.83 (300–823 K), offering a promising pathway for mid-temperature waste heat recovery and shedding light on vacancy engineering in thermoelectrics.

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• Electrical conductivity
• Energy materials
• Lattice dynamics
• Lattice thermal conductivity
• Phonons
• Seebeck effect
• Semiconductors
• Thermoelectrics
• Density functional calculations
• Resistivity measurements