On the positions of impurity modes in phonon spectra of metallic alloys

Positions of local and quasi-local impurity frequencies in metallic alloys have been considered. It has been shown that the differences between the frequencies calculated in the isotopic approximation and the experimental values can be explained not only by a change in the force constants (force shifts), as it is usually done, but also by a change in the effective mass of the oscillations (mass shifts). Comparison of the calculations with the experimental data demonstrates that the main role in the difference between the frequencies calculated in the isotopic approximation and the experimental values is played by the mass shifts rather than the force shifts, at least for local modes.