Lattice dynamics in perovskite-type layer structures. II. Rigid-ion model calculations for K2MnF4and Rb2MnCl4

For pt.I see ibid., vol.9, p.4213 (1976). A rigid-ion model with axially symmetric, short-range interactions originally developed for perovskite materials has been adapted to perovskite type layer structures by introducing new short-range force constants for the interlayer interactions. This model has been applied to KMnF 3 , K 2 MnF 4 and Rb 2 MnCl 4 . The adjustable parameters were fitted to the phonon frequencies at q=0 from infrared and Raman studies. It is shown that for KMnF 3 and the intralayer forces in K 2 MnF 4 the same values of the parameters can be used. This lattice dynamical comparison indicates that the short-range interactions are the same in the cubic perovskite and the related layer structure. The dominant interlayer interaction between alkali and halogen ions yields parameter values close to those in the corresponding alkali halides. This indicates that the interlayer bonding is more of the ionic type than of the van der Waals type.