Disorder effect on optical phonons

Optical phonons exhibit an asymmetric line shape in Raman light scattering if the size r₀ of region where the phonon interacts with an imperfection is smaller than the phonon mean free path l=a ω ₀/Γ . Here ω₀ and Γ are the phonon frequency and width, respectively, and a is the lattice parameter. The typical values of l are moderately large in the atomic scale a. This novel mechanism of line asymmetry is considered for imperfections of different types: point defects, dislocations, crystallite boundaries and strain fluctuations induced by interfaces. The dependence of the phonon frequency shift and width on the defect concentration is calculated, and the critical concentration at which the optical phonons can no longer be observed is defined.