Non‐Anharmonic Phonon Broadening in Real Crystals

The linewidth of the Raman mode can directly provide insight into phonon lifetime, a critical parameter governing the thermal, electrical, and optical properties of materials. Real crystals inherently deviate from ideal lattices due to isotopes, impurities, and defects, yet the explicit impact of these imperfections on non‐anharmonic phonon broadening remains elusive. Herein, temperature‐dependent Raman spectroscopy combined with first‐principles calculations unveils a significant temperature‐independent non‐anharmonic phonon broadening () in near‐ideal hBN‐encapsulated monolayer , primarily originating from isotopic mass disorder. Systematic studies on samples with progressively introduced defects further demonstrate that surface ripples, adsorbed impurities, and vacancies substantially contribute to in intralayer phonons. In contrast, interlayer shear modes in multilayer exhibit negligible sensitivity to uniform intralayer disorder, but are strongly affected by inhomogeneous interlayer imperfections. These findings establish a direct link between crystalline defects and non‐anharmonic phonon broadening, providing critical insights for phonon lifetime engineering and thermal management in functional semiconductor devices.