| Organic-inorganic metal halide hybrids are a class of composite crystal materials assembled by organic cations and metal halide anions,which have become a new generation of functional materials in the field of optoelectronic devices and have been extensively studied for their unique structure-property tunability.Meanwhile,organic-inorganic metal halide hybrids also act as the promising candidates to construct materials with structural phase transitions.Triggered by the external stimuli like temperature,the dynamic change of organic cations has great potential to promote the occurrence of phase transition,accompanied by the abrupt changes of physical properties near the phase transition temperature,such as thermal,dielectric,pyroelectric,piezoelectric and nonlinear optical properties.Such materials will hold promise for various applications spanning data storage,energy conversion and sensing.According to the structural characteristics of organic-inorganic hybrid metal halides,a series of phase transition materials have been successfully designed and synthesized.And the phase transition behaviors,variable-temperature structures and related physical properties have also been explored and included in this manuscript.The main contents are as follows:（1）Four organic-inorganic metal halide hybrids with high-temperature phase transitions,[（CH3）3NCH2Cl]2[Co Cl4]（1）、[（CH3）3NCH2Br]2[Co Br4]（2）、[（CH3）3NCH2Cl]2[Cu Cl4]（3）、[（CH3）3NCH2Cl][Cu Cl3]（4）,have been successfully synthesized.By means of modifying the halogen atoms in anions and cations and changing the coordination mode of anion skeleton,the phase transition temperature,phase transition behavior and dielectric properties were effectively regulated.In addition,a novel Sn（IV）-based zero-dimensional perovskite-type halide semiconductor,[（CH3）3NCH2I]2[Sn Cl6]（5）,with a reversible structural phase transition above room temperature,has been successfully prepared.The obvious steplike dielectric anomaly indicates its excellent switchable dielectric behavior.（2）We constructed two one-dimensional（1D）organic-inorganic perovskite materials,[（CH3）2NHCH2CH2OH][Cd Cl3]（6）and[（CH3）2NHCH2CH2OH][Mn Cl3]（7）,involving special hydrogen bonding interactions.In their crystal structures,strong hydrogen bonding interactions are not only found between the organic cations and inorganic chains（O-H——Cl）,but also among the[（CH3）2NHCH2CH2OH]+cations（N-H——O）to form 1D organic chains.Restricted by the two kinds of strong hydrogen bonds,the molecules in both compounds have the potential to adopt in the ordered state at low temperature.When the hydrogen bonds get weaker or even are destroyed as the external temperature rises,the organic cations may become disordered with dynamic motion,resulting in the occurrence of phase transition with remarkable switchable dielectric behaviors.（3）By introducing an electronegative halogen atom into the cation of[（CH3）4N][Pb I3]（8）,three organic-inorganic hybrid perovskite phase transition crystals,[（CH3）3NCH2X][Pb I3]（X=F（9）、X=Cl（10）、X=Br（11））,and a hybrid perovskite ferroelectric crystal,[（CH3）3NCH2I][Pb I3]（12）,were designed and synthesized.Among them,compound 8undergoes a reversible structural phase transition below room temperature.When a H atom on one methyl group of the cation is replaced by F,Cl,Br or I atoms successively,the phase transition temperatures of compounds 9-12 increase gradually with the increase of the substituted atomic number.And a ferroelectric phase transition occurs in the vicinity of room temperature with an Aizu notation of 2/m F2 in compound 12.Although the same anionic frameworks are detected in compounds 8-12,the strongest halogen bond energy of[（CH3）3NCH2I]——I and the largest volume of[（CH3）3NCH2I]+cation in compound 12 might be possible reasons for the stabilization of ordered organic cation arrangement and further reservation of is ferroelectricity at room temperature.This work provides an effective molecular design strategy for synthesizing novel organic-inorganic hybrid ferroelectrics. |
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