NHY coupling and localized vibration modes in MX compounds

The effects of NHY coupling on localized vibration modes around the excitations in MX compounds are investigated. It is shown that the localized modes in an isolated chain will be separated into two chains due to an NHY coupling. Some new modes of lattice vibration are found, which are achieved by the NHY coupling. These results can qualitatively account for the frequency shifts of some modes observed in the Raman and infrared absorption spectra.