Controlling Two-Step Phase Transitions and Dielectric Responses by A‑Site Cations in Two Perovskite-like Coordination Polymers

Two new perovskite-like coordination polymers, A₂[KFe­(CN)₆], were constructed by employing guanidinium and acetamidinium as A-site cations, respectively. Their cation-controlled two-step phase transitions as well as the relevant dielectric responses were uncovered by the combined techniques of the variable-temperature single-crystal X-ray structural analyses and dielectric measurements. With a similar size and shape, the A-site cations reveal similar two-step thermal-induced transitions on their motional dynamics, i.e., from a frozen order state to an in-plane rotational disorder state, and to a melt-like disorder state. However, the variation of the A-site cations on their symmetries and dipole moments makes noticeable impacts on the symmetry breaking, the critical temperatures, and the dielectric responses for the two-step structural phase transitions, i.e., the D_3h nonpolar guanidinium results in an R3̅c ↔ R3̅m ↔ Fm3̅m transition, whereas the C_2v polar acetamidinium results in a C2/m ↔ R3̅m ↔ Fm3̅m transition. Investigations of these two coordination polymers demonstrate a fine modulation on the phase transition behaviors and dielectric responses by changing the symmetries and dipole moments of A-site cations.