  Phonon spectra of solid substitutional solutions with finite concentrations
of impurities were analyzed on the microscopic level. The local phonon
densities of impurity atoms were calculated, in particular the formation of
quasilocal vibrations and their evolution with increasing of the concentration
of impurities were investigated. Modification of the local spectral densities
of atoms of the host lattice by impurities and manifestation of the phonon
Ioffe-Regel crossover (scattering of fast propagating phonons on
quasi-localized vibrations) were analyzed. It is shown that such scattering
causes the manifestation of the features such as "boson peak" in the phonon
spectrum of solid substitutional solutions. A commonality of the physical
nature of such singularities in disordered structures and van Hove
singularities in ideal crystals was established, notably, reducing of the group
velocity of acoustic phonons due to their scattering on slow phonons.
