Phonons in perovskite-type layer structures

Abstract

The authors present a rigid-ion model calculation for the high-temperature phase of (CH3NH3)2MnCl4 (MAMC) with the K2MnF4 structure (D4h 17). The short-range forces are assumed to be axially symmetric. The 11 disposable parameters of the model are fitted to experimental data, i.e. the frequencies of IR and Raman-active phonons, and to neutron scattering data. The high-temperature phase of MAMC is characterised by a disorder of the orientation of the organic ions. For their model, the authors assume each ion to be located at the symmetrised average position. Further, the organic ions are treated as rigid bodies since the authors are not interested in their internal modes. The results of the above model calculation confirm in MAMC the lattice dynamics are more or less three-dimensional and that the interlayer forces are of the same magnitude as the intralayer forces. The authors conclude that the three-dimensional lattice dynamical behaviour of these layered materials is due to the importance of long-range Coulomb forces in these ionic compounds.