Optical and Structural Anomalies in Potassium Ferrocyanide Trihydrate Crystals

Abstract

Potassium ferrocyanide trihydrate crystals grown from aqueous solution have been studied with polarizing microscope, X-ray diffraction methods, etching technique, and dielectric measurements at temperature region between 20° and −140°C. Various types of optical anomalies were observed, and the crystals were classified into five types by optical and X-ray investigations. Crystallographic data of monoclinic and tetragonal single crystals were measured. The various optic anomalies were clarified by twinning and parallel growth characteristic to the layer structure of the two modifications. Dislocation pits were observed by etching with a mixture of water and alcohol. Ferroelectric monoclinic phase, and the behaviors of tetragonal phase at low temperatures were studied by low temperature polarizing microscope, dielectric measurements and low temperature precession X-ray photographs. Teteragonal phase was shown to undergo a monotropic phase transition at about −55°C, and transform into a twinned monoclinic phase.