Infrared Dielectric Dispersion of Several Fluoride Perovskites

Measurements at room temperature of the infrared reflectivity, low-frequency dielectric constant (${\epsilon }_0$), and optical index of refraction ($n$) have been made on KCo${\mathrm{F}}_3$, KMn${\mathrm{F}}_3$, and RbMn${\mathrm{F}}_3$. The reflectivity data have been analyzed using Kramers-Kronig relations to obtain the dielectric dispersion from 50 ${\mathrm{cm}}^{-1\mathrm{}}$ to 4000 ${\mathrm{cm}}^{-1\mathrm{}}$. The frequencies of the three transverse optic (longitudinal optic) long-wavelength polar eigen-modes are 139 (168), 225 (293), 417 (500) ${\mathrm{cm}}^{-1\mathrm{}}$ for KCo${\mathrm{F}}_3$; 115 (143), 193 (268), 395 (479) ${\mathrm{cm}}^{-1\mathrm{}}$ for KMn${\mathrm{F}}_3$ and 113 (124), 197 (273), 369 (452) ${\mathrm{cm}}^{-1\mathrm{}}$ for RbMn${\mathrm{F}}_3$. ${\epsilon }_0=7.35, 8.28, 7.6$ and $n=1.47, 1.44, 1.53$ for KCo${\mathrm{F}}_3$, KMn${\mathrm{F}}_3$, and RbMn${\mathrm{F}}_3$, respectively. Mode strengths and damping were also measured. Low-temperature reflectivity measurements on antiferromagnetically ordered KCo${\mathrm{F}}_3$ were also performed.