Localized Modes of Vibrations in Ionic Crystals with an Isotopic Defect

摘要

An equation for the determination of the frequencies of localized modes of vibrations generated by an isotopic impurity in ionic crystals of NaC1-type is obtained by the Green functions method in harmonic approximation. It is shown that for the build-up of the vibrations of localized modes in lattices with similar values of ion masses, the impurity must be on an average twice lighter than the substituted ion. Dependence of the localized mode frequency on the impurity mass is investigated in detail for NaF. The frequencies of localized modes of vibrations of U-centres of H- and D- types for ten ionic lattices have been computed. The results of calculations are in agreement with the experimental data available. (Author)