Crystal Structures,solid-state Phase Transition And Dielectric Properties Of Co-Crystal Based On Hexakis(isothiocyanato)-Nickel(?)Anion

Electrically polarizable materials such as dielectric,piezoelectric,pyroelectric,and ferroelectric are important components of modern electrical and electronic devices.The study of moleculer-based electrically polarizable materials can not only provide new types of electrically polarizable materials,but also may have advantages such as easy preparation and light gravity.The perovskite-like CPs are usually essembled by the inclusion of a guest species into a well-matched three-dimensional host cage unit.These structurally designable and tunable compounds are apt to undergo structural phase transitions under external stimuli?such as temperature or pressure?,which may be good candidates for studying the polarization behavior and the related mechanism of guest ion/molecules in confined space.We have studied the orientation pattern and the molecular dynamics of polar guest cations confined in the three-dimensional cage-like coordination frameworks of some perovskite-like coordination polymers,in which the cage unit is formed by eight metal ion B²⁺possessing an octahedral coordination geometry,which connected by twelve end-to-end bridging ligand X^-.It was found that the motion of the guest cations is quite restricted by these relatively rigid coordination frameworks.Up to now,there are many researches on three-dimensional perovskite-like coordination polymers using small-size cationic templates,while it's difficult to find a well-matched three-dimensional cage-like coordination frameworks if the size of guest cations is increased.In this paper,a feasible strategy that the modification or replacement of such three-dimensional coordination frameworks by a relatively flexible supramolecular framework is presented to expand the confined space for the guest cations that reduces the rotation resistance to some extent.At the same time,it's expected that solid-state dielectric switches with highly adjustable switch temperatures?T_s,usually a temperature T_c at the solid-state structure phase transition?,which is related to an order-diorder structural phase transition associated with a dynamic dipolar change.With the above considerations in mind,using the rod-like thiocyanate anion as the terminal ligand and some different-size organic cation as the templates,we present three zero-dimensional ionic co-crystal compounds,namely?Me₃NH?₄[Ni?NCS?₆]?1?,?DMEA?₄[Ni?NCS?₆]?2?and?DMIPA?₄[Ni?NCS?₆]?3?,where DMEA,DMIPA represent N,N-dimethylethylamine,N,N-dimethylisopropylamine,respectively.Studies revealed that they all undergo a thermal induced ordered-disorder structural phase transition,in which compound 1undergoes an isostructural structural phase transition,and its switch temperatures?T_s?can be regulated by changing the frequency of the applied electric field.Variable temperature structure analysis and molecular dynamics simulation further reveal the intrinsic dielectric transfer mechanism from the microscopic molecular level.To sum up,through thermal analysis,single crystal structure analysis,variable temperature dielectric testing,and molecular dynamics simulation,we have gotten a better understanding about the dipole motion in crystalline environment.