Defect engineering offers a powerful route for tuning lattice thermal conductivity, yet a systematic comparison of the relative impact of distinct point defects and their extended counterparts in materials remains underexplored. In this work, we employ equilibrium molecular dynamics simulations to quantify the effects of vacancy, interstitial, substitutional, and void (extended) defects on lattice thermal conductivity in crystalline Si and Ge. Across all concentrations studied, vacancy and interstitial defects produce the greatest suppression of lattice thermal conductivity, whereas substitutional and void defects yield comparatively weaker scattering. The reduction in lattice thermal conductivity is strongly concentration dependent, with deviations between defect classes diminishing at low defect fractions. To elucidate the microscopic origins of these trends, we analyze changes in phonon transport through mass-weighted phonon density of states, revealing that defects which introduce large local strain fields or strong mass perturbations induce the most significant modifications in vibrational spectra. Our results provide a unified, comparative framework for understanding how different classes of defects in Si and Ge affect phonon scattering and thermal transport, enabling more rational design at the defect level for tuning thermal conductivity and thermal management applications.

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• Point defects
• Thermal conductivity
• Thermoelectrics