  It is well known that the optical branches of the dispersion curves of ionic
crystals exhibit a polaritonic feature, i.e., a splitting about the
electromagnetic dispersion line $\omega=ck$. This phenomenon is considered to
be due to the retardation of the electromagnetic forces among the ions.
However, the problem is usually discussed at a phenomenological level, through
the introduction of a macroscopic polarization field, so that a microscopic
treatment is apparently lacking. A microscopic first principles deduction is
given here, in a classical frame, for a model in which the ions are dealt with
as point charges. At a qualitative level it is made apparent that retardation
is indeed responsible for the splitting. A quantitative comparison with the
empirical data for LiF is also given, showing a fairly good agreement over the
whole Brillouin zone.
