Lattice dynamics, lattice instabilities and phase transitions in fluoride perovskites

The authors have measured the phonon dispersion curves of the fluoride perovskite materials KMnF3and KZnF3by means of coherent neutron inelastic scattering. The aim of this investigation was to provide the phonon spectrum of KMnF3which undergoes structural phase transitions and which has been studied by various methods in the last decade and, for comparison, of KZnF3in which no phase transitions occur. In addition they have found an anomalous temperature dependence of the R15' and M2modes in KZnF3which are the modes that soften near the phase transitions of KMnF3. This indicates that there is in general a tendency towards an instability against octahedra rotations in all fluoride perovskites. For the analysis of the phonon spectra, they have employed a lattice dynamical shell model under various conditions. In some of the models, they have successfully used interionic potentials which were derived from experimental data in other materials for some of the short-range interactions. Thus it could be shown that these potentials are also valid in the fluoride perovskites with different interionic distances. Furthermore, the experimental data are compared with the results of infrared spectroscopy and Raman studies at low temperatures as a test of the assignment of optically determined phonon frequencies to normal modes which are at the zone centre Gamma at low temperatures and at zone-boundary points in the cubic perovskite structure.