Optical phonons in ionic crystals with a diatomic unit cell

Optical phonons in an ionic crystal with two atoms per unit cell have been considered. The equation of relative motion of oppositely charged sublattices has been derived with the use of the concept of the self-consistent electromagnetic field. The frequency corresponding to transverse optical phonons and the expression for the frequency of the eigenmodes of the ionic crystal in terms of the physical characteristics of the crystal have been obtained from the condition of electrostatic equilibrium. The calculated values of the latter frequency agree with the experimental data.