Localized Lattice Vibration Modes Due to Substitutional Mass Defects in Sodium Iodide

Frequencies for local modes due to mass defects in NaI are computed on the deformation-dipole model for the perfect lattice. In this crystal local modes occur within the gap between the optical and acoustic branches of the frequency spectrum, as well as above the optical branches. Frequencies are computed for impurity ions of both signs, and of masses both greater and less than that of the host ions.