Far infrared transmission spectra and lattice vibrations of RbNiF3 and CsNiF3

The infrared absorption spectra of RbNiF8 and CsNiF3, both with a perovskite-like structure, have been measured in the region 500-50 cm(-1) at room temperature and at liquid nitrogen temperature. The observed bands can be classified into three groups, corresponding to the three absorption bands in perovskite. These bands can reasonably be assigned on the basis of the crystal symmetry of the investigated compounds. A normal coordinate analysis of a crystal as a whole has been performed, where the interatomic force constants between the atom pairs within a distance of 3.5 angstrom are taken into account. It was shown that the normal coordinate treatment of the optically-active vibrations on the basis of a molecular dynamics model is useful for the interpretation of the lattice vibrations in these fluorides. The effects of the structural differences of the lattice vibrations are discussed.