COOPERATIVE JAHN-TELLER EFFECTS IN THE PHASE TRANSFORMATIONS OF $Cs_{2}LiFe(CN)_{6}$

Abstract

We have been interested in describing the structural phase transformations of crystals containing transition metal hexacyanide ions in terms of phonon modes. The phase transitions for the $Cs_{2}LiM(CN){6} \ (M = Cr^{+3}, Fe^{+3}, Co^{+3}, and \ Ir^{+3})$ salts have been probed using Raman scattering and X-ray crystallography. Phase changes of the $Cr^{+3}, Co^{+3},\ and Ir^{+3}$ salts involve external phonons exclusively. However, the iron salt, which is isostructural with the other members of the dicesium lithium series at room temperature, exhibits phase transformations involving both external and internal modes of vibration. In effect, the $Fe(CN){6}^{-3}$ ion is distorted from $O_{h}$ symmetry in going through the phase transition at $221^\circ K$ (the $Fe(CN){6}^{-3}$ ion occupies a site of $O{h}$ symmetry in the Fm3m high temperature cell). The $221^\circ K$ structural distortion occurs along $F_{1g}$ symmetry phonon(s) which involve Fe-C-N deformation and rotation of the hexacyanide moiety. The unusual phase transformations exhibited by the $Cs_{2}LiFe(CN)_{6}$ salt have been attributed to a cooperative interaction between the Jahn-Teller instability of the ferricyanide ion and the external phonon instabilities which dominate the phase transformations of the related $Cr^{+3}, Co^{+3} and \ Ir^{+3}$ salts.