Mode‐Resolved Phonon Dynamics Under Chemical Pressure in SnTe Thermoelectrics

ABSTRACT Chemical pressure is used to tune lattice thermal conductivity ( κ l ) in thermoelectric materials, yet its effect on phonon transport remains uncovered. Here, Sn 0.95 M 0.05 Te (M = Sn, Mn, Ge, Sm, and Pb) is used to investigate how substitution‐induced chemical pressure modifies phonon dynamics in SnTe. Mapping refined lattice parameters onto an equivalent pressure scale shows that Sn 0.95 M 0.05 Te deviates from the elasticity‐based κ l trend defined by physical pressure, indicating a nonuniform influence of chemical pressure on different phonon modes. Terahertz time‐domain spectroscopy, Raman spectroscopy, and Brillouin spectroscopy resolve the Γ‐point optical mode, L‐point optical modes, two‐phonon Raman features, and long‐wavelength acoustic phonons. These measurements, combined with first‐principles lattice dynamics and spectral energy density analysis, show that chemical pressure reshapes the phonon spectral functions of low‐energy optical modes through peak shifts and linewidth broadening. This spectral‐function reshaping enhances their overlap with acoustic heat carriers, promotes acoustic–optical scattering, and shortens acoustic phonon lifetimes. These results identify low‐energy optical phonons as the key degrees of freedom through which chemical pressure controls thermal transport in SnTe.