Far-infrared Reflection Spectra, Optical and Dielectric Constants, and Lattice Vibrations of Some Fluoride Crystals

The reflection spectra of the crystals of the NaCl structure (LiF, NaF, and NiO), the fluorite structure (CaF2, BaF2, and CdF2), and the rutile structure (MgF2 and MnF2) were measured. The optical and dielectric constants were derived from the measured reflectivities using the Kramers-Kronig transformation. The absorption indices (k) for these crystals are rather large and the peak values of the imaginary dielectric constants amount to 30.0 or more. However, unlike the ferroelectric substances such as oxide perovskites and rutile, these fluoride crystals do not show any anomalous dielectric behaviour. From the real and imaginary dielectric constants ε′(ν) and ε″(ν), the transverse and longitudinal frequencies for the lattice modes are determined. These frequencies are reasonably interpreted on the basis of the crystal structures and satisfy the Lyddane-Sachs-Teller relation. The effective charge on the positive and negative ions and the force constant associated with the short-range elastic force against the ion displacement are obtained for the cubic crystals.