Temperature-Induced Structural Phase Transitions in Two New Postperovskite Coordination Polymers

We presented two new ABX3 postperovskite coordination polymers, (C5H13NCl)[M(dca)3] (dca = N(CN)2–, M = Mn2+ for 1 and Cd2+ for 2), as well as their phase transition behaviors disclosed by using differential scanning calorimetry measurements, variable-temperature single-crystal X-ray analyses, dielectric measurements, and Hirshfeld surface analyses. Both 1 and 2 have same structure (Cmcm) at room temperature, in which their A-site organic cations are fourfold disordered about two mirror planes and undergo order–disorder phase transition mainly caused by the freezing of A-site cations upon cooling. The different metal ions endow them distinct structural flexibility, resulting in different phase transition behaviors and dielectric responses; that is, the smaller and coordinatively rigid Mn2+ ion results in two-step Cmcm (Z = 4) ↔ Pbcm (Z = 4) ↔ Pbca (Z = 8) transitions accompanied with obvious dielectric relaxation, whereas the larger and coordinatively flexible Cd2+ ion results in one-step Cmcm (Z = 4) ↔ Pbca (Z = 16) transition accompanied with a sharp dielectric switching. This study well demonstrates the tunability of postperovskite, as a new kind of host–guest model, for modulating the phase transition and relevant switching physical properties.