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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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The basic equations of three-body force shell model have been corrected for the ionic charge parameter Z. This correction also modifies the expressions for the shell and core charge parameters. Our model has been applied to study the phonon dispersion curves, Debye temperatures variation, combined density of states and two-phonon Raman scattering spectra of wustite. This approach has been found successful in interpreting the complete lattice dynamics of FeO only when both the ions are considered to be polarizable. There is good scope for the fresh determination of positive ion polarizability and Debye temperatures variation. It is concluded that the presence of magnetic and anharmonic interactions have no effect on the lattice dynamics of FeO in its paramagnetic phase.

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