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A new model for studying the lattice dynamics of ionic and semiconducting crystals has been developed by incorporating the effects of van der Waals interactions (VWI), free-carrier doping (FCD) and three-body interactions (TBI) in the framework of the both-ions polarizable rigid shell model (RSM). This model (VFTSM) has been applied to calculate the phonon dispersion relations, Debye temperature variations and two-phonon infrared and Raman spectra of IV-VI ionic semiconductors (PbS, PbSe and PbTe). The excellent agreement between theory and experiment clearly shows that our model is superior to earlier models in explaining the dynamical behaviour of these crystals.

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