Design,Synthesis And Properties Of Three-dimensional Molecular Ferroelectricity

It is well known that CH₃NH₃Pb I₃ is a high-performance and low-cost light-harvesting material for photovoltaic devices,which shows great application prospects for the development of next-generation solar devices.The structures of molecular perovskites have recently attracted a great deal of academic attention because of their potential to yield unique physical properties.However,despite great efforts,so far reported perovskite-like molecular ferroelectrics are basically one-or two-dimensional,Molecular ferroelectrics with three-dimensional（3D）perovskite structures are still rare.Therefore,we design and synthesize quasi-spherical molecules 1,5-diazabicyclo[3.2.2]nonane（1,5-3.2.2-dabcn）and 1,4-diazabicyclo[3.2.2]nonane（1,4-3.2.2-dabcn）using 1,4-diazabicyclo[2.2.2]octane as a template.Four kinds of three-dimensional organic-inorganic hybrid perovskites are synthesized by reacting 1,5-3.2.2-dabcn and 1,4-3.2.2-dabcn with Rb X（X=Cl,Br,I）and the corresponding acids in a ratio of 1:1:2 that are[1,5-3.2.2-dabcn]Rb Br₃（1）,[1,5-3.2.2-dabcn]Rb I₃（2）,[1,4-3.2.2-dabcn]Rb I₃·H₂O（3）and[1,4-3.2.2-dabcn]Rb Br₃（4）.Infrared spectroscopy（IR）and powder X-ray diffraction（P-XRD）verify the purity of the crystals,while differential scanning calorimetry（DSC）,dielectric,second-order nonlinear optics and temperature-variable single crystal X-ray diffraction verify the ferroelectricity of the crystal.The results show that compounds 1～3 are all potential three-dimensional organic-inorganic hybrid perovskite ferroelectric materials.Among them,compound1 is tested by piezoelectric force microscopy（PFM）and the results prove that compound 1 is an organic-inorganic hybrid perovskite ferroelectric.Compound 4 is a three-dimensional organic-inorganic hybrid perovskite dielectric material.From the phase transition point of compounds 1 and 2,T_c（1）=342 K,T_c（2）=293 K,so T_c（1）>T_c（2）,it can be concluded that the same organic amine reacts with Rb X（X=Br,I）to produce products with the same structure,and its phase transition temperature is T_c（ARb Br₃）>T_c（ARb I₃）.The reason for the difference of the phase transition point is analyzed by the crystal structure before and after the phase transition point T_c:it is related to the bond energy of the metal salt and the intermolecular force that forms the hydrogen bond.The cavities formed by the 3D Rb-Cl inorganic framework cannot accommodate 1,5-3.2.2-dabcn and 1,4-3.2.2-dabcn because they are not within the range of the Goldschmidt tolerance factor,so the target 3D compound[1,5-3.2.2-dabcn]Rb Cl₃ and[1,4-3.2.2-dabcn]Rb Cl₃ are not obtained.According to the single crystal test results of compounds 1～4,we find that compound 4 forms a larger three-dimensional framework through face-sharing,which could accommodate organic amines with larger diameters.Because Cesium is in the same main group as Rubidium,in aqueous hydrobromic acid,I react 1,5-3.2.2-dabcn and 1,4-3.2.2-dabcn with Cesium Bromide,respectively,to obtain[1,5-3.2.2-dabcn]Cs Br₃（5）and[1,4-3.2.2-dabcn]Cs Br₃（6）.The purity of the crystal is verified by infrared spectroscopy（IR）and powder X-ray diffraction（P-XRD）,and its ferroelectric properties are characterized by DSC,dielectric,SHG and variable temperature single crystal X-ray diffraction.The results show that compound 5 is a potential three-dimensional organic-Inorganic hybrid perovskite ferroelectric material,compound 6 is a three-dimensional organic-inorganic hybrid perovskite dielectric material.To design and synthesize 3D metal-free perovskite ferroelectric materials,We react the quasi-spherical molecules 1,5-3.2.2-dabcn and 1,4-3.2.2-dabcn with NH₄X（X=Br,I）and the corresponding acids in a ratio of 1:1:2,respectively,and obtain four three-dimensional Metal perovskites which are[1,5-3.2.2-dabcn]NH₄Br₃（7）,[1,5-3.2.2-dabcn]NH₄I₃（8）,[1,4-3.2.2-dabcn]NH₄Br₃（9）and[1,4-3.2.2-dabcn]NH₄I₃·H₂O（10）.Their purity can be characterized by both infrared spectroscopy and powder XRD diffraction.By DSC,dielectric,SHG and temperature-variable single-crystal X-ray diffraction,it is proved that compounds 8and 10 are potential 3D metal-free perovskite ferroelectrics,while compounds 7 and9 are 3D metal-free dielectric Materials.It is worth noting that,from the crystal structure,the three-dimensional structure of compound 9 is a three-dimensional framework formed by NH₄Br₆ through corner sharing and face sharing.This three-dimensional framework has larger pores and can accommodate larger organic amine molecules.According to DSC test results,T_c（7）=337K,T_c（8）=297K.By observing the T_c of the above two single crystals,it is found that T_c（7）>T_c（8）.We conclude that when the same organic amine reacts with NH₄X（X=Br,I）to produce products with the same structure,the phase transition sequence of the products is T_c（ANH₄Br₃）>T_c（ANH₄I₃）.This conclusion is the same as that of the phase transition temperature of single crystals obtained by the reaction of organic amines with Rb X（X=Br,I）.In conclusion,we reduce the symmetry of 1,4-diazabicyclo[2.2.2]octane to design and synthesize quasi-spherical molecules 1,5-3.2.2-dabcn and 1,4-3.2.2-dabcn.They are react with Rb X（X=Br,I）,Cs Br and NH₄X（X=Br,I）,respectively,to obtain 6 kinds of 3D organic-inorganic hybrid perovskite compounds and 4 kinds of3D metal-free perovskite materials.The properties of these 10 crystals are explored through various characterizations methods.It turns out that reducing the symmetry of organic amines has a crucial role in the design of ferroelectrics.