The discovery of ultra-low lattice thermal conductivity has always been an urgent task to break through the thermoelectric performance of environment-friendly SnTe materials. As an effective lattice modulation strategy, lattice softening without deteriorating electrical transport has attracted much attention. Herein, we systematically compare and explore the electron-phonon transport characteristics of Q-doped SnTe-GeTe alloys (Q = Bi, Sb, and Ag), especially the visible lattice softening at sound velocity. The high solid-solution of GeTe significantly reduces the sound velocity, and the apparent strong phonon softening is the major contribution to the low lattice thermal conductivity. However, additional Sb/Bi/Ag doping exhibits various modulations on its "softening". Bi and Ag with large ionic radii easily destroy the crystal structure and restore the sound velocity close to that of pristine SnTe, while Sb with smaller ionic radius maintains the sound velocity. The Debye-Callaway model quantifies phonon softening and defect scattering, elucidating the exotic lattice thermal conductivity distortion. Additionally, the compensated band convergence and carrier concentration modulation synergistically optimize the electrical transport. The trade-off between lattice softening and electronic band modulation ultimately leads to the zT values of similar to 1.0,similar to 1.1, and similar to 0.8 in Sn0.66Ge0.3Bi0.04Te, Sn0.66Ge0.3Sb0.04Te, and Sn0.66Ge0.3Ag0.04Te, respectively. This work not only confirms the feasible manipulation of high-concentration GeTe alloying on the lattice softening of SnTe, but also emphasizes the significant modulation effect of trace aliovalent ion doping, providing a useful reference for optimization of lattice thermal conductivity.
