Infrared Dielectric Dispersion of Several Fluoride Perovskites

Measurements at room temperature of the infrared reflectivity, low-frequency dielectric constant (ε₀), and optical index of refraction (n) have been made on KCoF₃, KMnF₃, and RbMnF₃. The reflectivity data have been analyzed using Kramers-Kronig relations to obtain the dielectric dispersion from 50 cm^-1 to 4000 cm^-1. The frequencies of the three transverse optic (longitudinal optic) long-wavelength polar eigen-modes are 139 (168), 225 (293), 417 (500) cm^-1 for KCoF₃; 115 (143), 193 (268), 395 (479) cm^-1 for KMnF₃ and 113 (124), 197 (273), 369 (452) cm^-1 for RbMnF₃. ε₀=7.35, 8.28, 7.6 and $n=1.47, 1.44, 1.53$ for KCoF₃, KMnF₃, and RbMnF₃, respectively. Mode strengths and damping were also measured. Low-temperature reflectivity measurements on antiferromagnetically ordered KCoF₃ were also performed.