Optical Phonons in Ionic Crystals in the Presence of Surfaces or Defects
We have investigated the properties of long wave optical phonons in ionic crystals in the presence of surfaces or defects. Because of the existence of long range Coulomb forces the localized phonon modes in these crystals differ in many respects from those appearing in crystals in which the atoms interact through short range forces only. Whereas in the latter case the displacement amplitudes decay exponentially or even faster with increasing distance from the surface or the impurity site, it is found that in the presence of long range forces the localized optical modes usually decay much more slowly.
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