Structural Phase Transitions And Dielectric Properties Of The Perovskite-like Azido Coordination Polymers Containing Polar Guest Cations

The polarization behavior of the guest ions or molecules in confined space and their response to external stimulus are the source of some molecular-based functional materials with unique physical properties. As a type of coordination-based host-guest compounds, perovskite-like coordination polymers（CPs） are good candidate for the study of the polarization behavior of the guest cations in the well-matched cage-like host frameworks. For these perovskite-like CPs, they can undergo reversible solid-solid structural phase transition upon thermal stimulus, which is mainly caused by the variable rotation mode of the guest cation.In this paper, using azide as the bridging ligands, we present two new perovskite-like azido CPs with polar cation as the guest, namely [（CH₃）₂NH₂][Cd（N3）3]（1） and [（CH₃）₃NH][Cd（N₃）₃]（2）. In this thesis, we focus on the mechanism of the structural phase transition and the related polarization effect. From the thermodynamic point of view, the phase transition types of 1 and 2are first-order and second-order, respectively, which means that the inherent phase transition mechanism of them differs essentially. The much larger Ea value of 2 can be mainly ascribed to the heavier weight and bigger volume of its [（CH3）3NH]+guest cation in the confined space. The dielectric relaxation can be obviously observed in both compounds. The dielectric measurements of 1 show rarely observed two-stage switchable dielectric relaxations, revealing that the molecular dynamics of the polar cation guest are well controlled by the variable confined space of the host framework. In addition, using dicyanamide（Dca） as the building block, we obtain a simple copper（II） complex with N,N-dimethyl-ethylenediamine（Dmda） as the auxiliary ligand, namely[Cu（Dca）2（Dmda）]（3）. Its temperature-dependent structures and the polarization effect are studied.The dimer units in 3 are further assembled through two hydrogen bonding interactions to form two types of supramolecular chains. A notable lattice deformation in 3 is mainly caused by the deformations of the supramolecular chains during the heating process.Through the studies on the solid-state structures and phase transition mechanism of these compounds, we have a more in-depth understanding on the polarization effect of these molecule-based compounds.