FAR INFRARED SPECTRA AND LATTICE VIBRATIONS OF PEROVSKITE FLUORIDES $AMF_{3}$ OF TRANSITION METAL IONS

Abstract

Far infrared transmission spectra have been observed at room and liquid nitrogen temperatures for the powder samples. Reflection spectra have been measured for the crystalline samples and optical constants have been obtained from a Kramers-Kronig analysis. $KNiF_{3}$, $RbNiF_{3}$ (high pressure phase), $RbMnF_{3}$ and $RbFeF_{3}$ have cubic perovskite structure ($O{_{h}}^{1}$). $RbNiF_{2}$ and $CsMuF_{3}$ (atmospheric pressure phase) have 6L hexagonal structure ($D{_{6h}}^{4}$) $CsNiF_{3}$ of 2L structure ($D{_{6h}}^{4}$) transforms to the 9L structure ($D{_{3d}}^{9}$) under high pressure. $NaNiF_{3}$ is not uniaxial but it may have the orthorhombic structure of $GdFeO_{z}$ type ($D{_{2h}}^{16}$). Vibrational spectra of these crystals exhibit different features, which reflect the structural variations. On the basis of the dynamical model of crystal, the interionic potential constants corresponding to the $M^{2+}\ldots F^{-}$ and $A^{+}\ldots F^{-}$ interactions are evaluated to be 0.5 $\sim$ 0.8 md/{\AA} and ca. 0.1 md/{\AA}, respectively, from the observed optically active transverse frequencies.