Pyrrolidinium-Based Cyanides: Unusual Architecture and Dielectric Switchability Triggered by Order-Disorder Process

Two three-dimensional metal-organic compounds of the formula Pyr₂KM(CN)₆, where M = Co, Fe and Pyr = pyrrolidinium ((CH₂)₄NH₂⁺), have been found to crystallize at room temperature in a monoclinic structure, space group P2₁/c. They are cyano-bridged compounds with an unprecedented type of architecture containing pyrrolidinium cations in the voids. The materials have been investigated by X-ray diffraction, dielectric, and spectroscopic methods as a function of temperature in order to determine their properties and the mechanism of the reversible phase transitions occurring at ca. 345-370 K. The phase transitions in both crystals are first order and are associated with a symmetry increase to a rhombohedral structure (space group R3̅m) as well as a significant disorder of organic cations above T_c. On the basis of Raman scattering and IR spectroscopy it has been assumed that the phase transition in both crystals is triggered by thermally induced pseudorotation of the organic cation and large out-of-plane motions of its atoms followed by a "click-in" of the cyanide bridges. The materials have been proposed as possible switchable dielectrics due to their respective high differences in dielectric permittivities across the phase transition.