Nonadiabatic effects in the phonon spectrum of metal compounds

It is shown that nonadiabatic corrections to the dispersion law of optical phonons in the region of small wave vectors in the case of branches, for which the vibration with a zero wave vector is not accompanied by the appearance of a dipole moment in the ionic lattice, are significant for all possible directions of the wave vector. If a dipole moment arises, nonadiabatic corrections reach a noticeable value only for the wave-vector directions that are almost perpendicular to the direction of the dipole moment.