Transverse and longitudinal lattice frequencies and interionie potential in some AMF 3 perovskite fluoride crystals

Abstract

The i.r. reflection spectra of the crystals RbMnF 3, RbFeF 3, KMgF 3, and CsMnF 3 were measured. The optical and dielectric constants were derived from the measured reflectivities. The transverse [ν(T)] and longitudinal [ν(L)] frequencies of the cubic crystals BbMnF 3, RbFeF 3 and KMgF 3 were obtained, from the real and imaginary dielectric constants, ϵ'( v) and ϵ''( v). The calculation of the ν(T) and ν(L) frequencies for y → (0, 0, 0) was made, on the basis of a simple rigid ion model which includes effective ionic charges and interionie repulsive force constants. The least squares calculation determined a set of potential constants and effective ionic charges, using the observed values of both the ν(T) and ν(L) frequencies. This simple rigid ion model may explain satisfactorily the lattice vibrations of AMF 3 cubic crystals. The short range force constant associated with the metal ion to fluorine ion indicates that this bond contains some amount of covalent character.