The cation-dependent structural phase transition and dielectric response in a family of cyano-bridged perovskite-like coordination polymers

A family of cyano-bridged perovskite-like coordination polymers (CPs), [(CH3)(n)NH4-n](2)[KFe(CN)(6)] (n = 1 (1), 2 (2), 3 (3), and 4 (4)), were synthesized and characterized. The differential scanning calorimetry measurements and variable-temperature single-crystal X-ray structural analyses revealed that, owing to the deformation of the host framework as well as the dynamic transition of the guest cation between the static/ordered and dynamic/disordered states, the four CPs undergo structural phase transitions (at 429, 226, 316, and 350 K, respectively) with the symmetry breakings dependent on the symmetries of the encapsulated guest cations. The modulated differential scanning calorimetry measurement suggested that the phase transitions of 1 and 3 have more striking kinetic processes related to their drastic deformation of the host framework as well as a very significant alteration of the host-guest interaction during the phase transition. Moreover, accompanying the transitions between low- and high-temperature phases, the step-like transitions between low and high dielectric states were observed in 1-3, and the corresponding change in amplitude of the dielectric constant is dependent on the total dipole moment of each cage in the high-temperature phase. The investigation on these host-guest CPs deepens the understanding of the relationship between the dipole moment of guest cations and the dielectric behaviour of materials, shedding light on the search for new switchable molecular dielectrics.