  We consider the dynamics of an ionic crystal with a single impurity in the
vicinity of the polariton resonance. We show that if the polariton spectrum of
the host crystal allows for a gap between polariton branches, the defect gives
rise to a novel kind of local states with frequencies within the gap. Despite
the atomic size of the impurity we find that new local states are predominated
by long-wavelength polaritons. The properties of these states are shown to be
different from the properties of the well-known vibrational local states. The
difference is due to the singular behavior of the density of states of
polaritons near the low-frequency boundary of the polariton gap. Assuming cubic
simmetry of the defect site we consider a complete set of the local states
arising near the bottom of the polariton gap.
