Infrared Studies of Some Perovskite Fluorides. I. Fundamental Lattice Vibrations

The reflectance spectra of the cubic fluoride perovskites KMgF3, KMnF3, KCoF3, KNiF3, RbMnF3, and mixed crystals of K (Mg0.8:Ni0.2) F3 and K (Mg0.5:Ni0.5) F3 have been measured from 4000−40 cm−1 at 300° and 85°K. The spectra of all crystals at both temperatures have been fitted by means of a classical dispersion analysis to yield the frequencies, oscillator strengths, and damping constants of the three ir active lattice vibrations. An assignment of the normal mode frequencies to particular vibrations has been made. The high- and low-frequency dielectric constants of these materials have been determined from the reflectance measurements. The isotropic polarizabilities of Mg2+, Mn2+, Co2+, Ni2+, and Rb+ have been calculated. The transverse effective charge at 300°K has been obtained in order to determine the strength of second-order dipole moments due to charge deformation during lattice vibrations.