Mössbauer Test for a Lattice-Dynamical Model for the Ferroelectric Phase Transition in Potassium Ferrocyanide

A lattice-dynamical model is proposed for the ferroelectricity in potassium ferrocyanide, in contrast to the previously accepted order-disorder model. This proposed model associates the dielectric anomaly with a mode instability where ωTO→0 at q→0 in the [101̄] crystallographic direction. Measurements of the M\"ossbauer fraction as well as the dielectric constant were carried out simultaneously on a single crystal, as a function of temperature, to provide identification of the phase transition and as a test for the dynamics of the iron framework through the transition. The results show large changes in the M\"ossbauer fraction in the vicinity of the transition temperature perpendicular to the [010] direction only. This is compatible with our lattice-dynamical treatment. This model is further supported by a structural discussion, analysis of acoustic data, and the shifts of the transition temperature on deuteration and atomic substitution. The M\"ossbauer and substitutional data presented are incompatible with the order-disorder theory.