Suppressing lattice thermal conductivity ( $\kappa _{\mathrm{lat}}$ ) is pivotal for thermoelectric efficiency. While traditional strategies rely heavily on phonon scattering from mass- and size-mismatches, we demonstrate a robust $\kappa _{\mathrm{lat}}$ suppression mechanism driven by dopant-induced lattice stiffness modulation. Through a comparative analysis of p-type (Mn) and n-type (Co, Ir) doping in the β-FeSi2 model system, we show that Co and Ir doping significantly reduce $\kappa _{\mathrm{lat}}$ . Notably, Co doping achieves a ∼71% reduction at 300 K even without significant mass and size contrast. By correlating transport data with neutron powder diffraction, heat capacity, and Raman spectroscopy, we reveal anomalous lattice expansion, a substantial reduction in Debye temperature, and marked vibrational redshift and broadening. These systematic changes provide strong evidence for atomic-scale lattice softening and a fundamental weakening of interatomic force constants, which synergistically lower phonon group velocities and amplify anharmonic scattering. Our findings establish lattice stiffness manipulation as a powerful strategy for thermal management, offering a distinct design pathway beyond traditional mass- and strain-fluctuation models.
