Phase transition in the extreme: a cubic-to-triclinic symmetry change in dielectrically switchable cyanide perovskites

Two novel three-dimensional metal-organic compounds of the formula FA2KM(CN)6, where M=Co, Fe and FA=formamidinium (CH(NH2)2+), have been found to crystallize in a perovskite-like architecture. They have been investigated by X-ray diffraction, dielectric and spectroscopic methods as the function of temperature in order to determine the interactions in the crystal and the mechanism of the phase transitions occurring at ca. 321 K upon heating. The phase transitions in both crystals are of first order and originate from ordering the formamidinium cations below TC. It seems to be the strongest symmetry reduction (cubic-to-triclinic) upon temperature decrease found among known metal-organic frameworks. The materials have been proposed as possible switchable dielectrics with a convenient near-room-temperature transition temperature.