Mossbauer investigation of the intra- and inter-molecular dynamics of K3Fe(CN)6—electron-phonon instability at ∼130 K

Abstract

The Mössbauer data for K₃Fe(CN)₆ are analyzed in terms of inter- and intra-molecular vibrations. The characteristic Einstein temperatures obtained are θ_inter 57 K and θ_intra 650 K; the latter is in good agreement with i.-r. spectra. A comparison with the results for K₄Fe(CN)₆ and K₄Fe(CN)₆ . 3H₂O shows that the change in the nominal valency of the central iron atom affects mainly the intermolecular vibrations. In contrast, the three water molecules (K₄Fe(CN))₆ . 3H₂O) affect significantly the intramolecular dynamics indicating that they are bonded to CN groups forming an extended [Fe(CN)₆(H₂O)₃]^4– complex. Thermal population of a low lying electronic energy level, due to a small rhombic distortion of the crystal field, may cause an “electron-phonon instability” which will explain the observed ln ƒ(T) anomalies as well as the reported anomalies in the thermodynamic measurements and paramagnetic susceptibilities of K₃Fe(CN)₆ at 130 K.