Lattice dynamics in perovskite-type layer structures. I. FIR and Raman studies on K2MnF4and Rb2MnCl4

The far-infrared reflectivity of K 2 MnF 4 and Rb 2 MnCl 4 single crystal was studied for E perpendicular to c and E approximately=c at several temperatures. By means of a Kramers-Kronig analysis, the frequencies of the long-wavelength infrared-active phonons were determined. In addition, Raman spectra of Rb 2 MnCl 4 were measured. Two peaks were found in both of the x(zz)y and x(zy)z spectra. From these maxima the frequencies were deduced for phonons with the Gamma 1 + (A 1g ) and Gamma 5 + (E g ) symmetries, respectively. The frequency dependence in K 2 MnF 4 and Rb 2 MnCl 4 was studied between 13K and room temperature. This investigation was performed with the intention of obtaining a complete set of experimental data for lattice dynamical considerations especially with respect to the intralayer and interlayer forces in K 2 MnF 4 and Rb 2 MnCl 4 .