Long-wave longitudinal optical phonons as quanta of electromagnetic field in cubic ionic crystals

We consider long-wave high-frequency upper phonon-polaritons and longitudinal optical phonons in diatomic ionic crystal. The model of free ions which are polarized in self-consistent electromagnetic field in the dielectric environment is used. A wave equation for the electric field in a medium is obtained that gives standard dispersion law for upper branch phonon-polaritons and longitudinal optical phonons. It is shown by comparison with tabular data that the frequency of longitudinal optical phonons coincides with an ion plasma frequency with the reduced mass of the crystal cell and the factor with high-frequency dielectric constant, arising through the use of the Lorentz relation.