Raman Scattering Due to Disorder-Induced Polaritons
  The selection rules for dipole and Raman activity can be relaxed due to local distortion of a crystalline structure. In this situation a dipole-inactive mode can become simultaneously active in Raman scattering and in dipole interaction with the electromagnetic field. The later interaction results in disorder-induced polaritons, which could be observed in first-order Raman spectra. We calculate scattering cross-section in the case of a material with a diamond-like average structure, and show that there exist a strong possibility of observing the disorder induced polaritons. 
Lev I. Deych, M. Erementchouk, and V.A. Ignatchenko