Inthetheorycalculationoflatticevibration,oneacousticandthreeopticalbrancheswerefoundtocomposethephononvibratingspectrum.Someisolatedmodeswithfrequencieslyingoutsidethecontinuumbrancheswillariseunderthedefectstates.TheselocalmodelresultsinthesharppeaksintheinfraredabsorptionandRamanspectra.Fromcalculationoftheinfraredabsorption,thelocalmodewiththeinfraredactivityisobtainedintheinfraredabsorptionspectrumofMXcompounds.