Mode-Resolved Phonon Dynamics Under Chemical Pressure in SnTe Thermoelectrics

Chemical pressure is used to tune lattice thermal conductivity (κ_l) in thermoelectric materials, yet its effect on phonon transport remains uncovered. Here, Sn_0.95M_0.05Te (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_0.95M_0.05Te 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.