Concept of local polaritons and optical properties of mixed polar   crystals
  The concept of local polaritons is used to describe optical properties of mixed crystals in the frequency region of their {\it restrahlen} band. It is shown that this concept allows for a physically transparent explanation of the presence of weak features in the spectra of so called one-mode crystals, and for one-two mode behavior. The previous models were able to explain these features only with the use of many fitting parameters. We show that under certain conditions new impurity-induced polariton modes may arise within the {\it restrahlen} of the host crystals, and study their dispersion laws and density of states. Particularly, we find that the group velocity of these excitations is proportional to the concentration of the impurities and can be thousands of times smaller then the speed of light in vacuum. 
Lev I. Deych, Alexey Yamilov, and Alexander A. Lisyansky