Study On The Effect Of Molecular Symmetry On The Phase Transition Of Solid Structures

Many physical properties of crystalline materials,such as ferroelectricity and piezoelectric properties,depend on specific phases.Therefore,adjusting the structural phase transition properties of crystals is of great significance for achieving specific functions.In recent years,organic-inorganic hybrid phase-transition materials have been found to have important potential applications in emerging fields such as flexible devices and biological built-in devices.Adjusting the phase transition properties of these materials is the key to the development of these materials.Chemical and structural characteristics such as fluorogenic effects and molecular chirality have been successfully used to tune the properties of structural phase transitions.Due to the rich and diverse structure of organic-inorganic hybrid materials,it is of important academic value to construct new phase transition systems and explore new factors that affect phase transition.Therefore,several series of compounds with structural phase transitions has been constructed in this thesis,and the effect of molecular symmetry on structural phase transitions is studied.The main research work is as follows:（1）A charge transfer compound（TPY）（NP）Sb F₆（TPY is a tropylium cation,NP is a neutral naphthalene molecule）（1）was constructed.It was found that the planar cation and octahedral anion have the characteristics of easy rotation,allowing for a structural phase transition.Using 1 as a reference structural model,a series of charge transfer compounds were synthesized using neutral molecules and anions with lower symmetry,including（TPY）（DNP）Sb F₆（DNP is 1,4-dimethylnaphthalene）（2）,（TPY）₂（Fy）₃（Sb F6）₂（Fy is phenanthrene）（3）,（TPY）（MNP）Sb F₆（MNP is 2-methylnaphthalene）（4）,（TPY）₂（NP）₃（Ga Cl₄）₂（5）and（TPY）₂（Fy）₃（Ga Cl₄）₂（6）.It was found that the decrease in molecular symmetry allowed the crystal to crystallize in a lower symmetry space group,while significantly increases the phase transition temperature.（2）A one-dimensional antimony（Ⅲ）based chloride（Q）₂Sb Cl₅（Q is the quinuclidinium cation）（7）was constructed,realizing a reversible high-temperature solid-solid phase transition.Using 7 as a reference structure model,（3-O-Q）₂Sb Cl₅（8）was constructed using a less symmetrical 3-O-Q（3-quinuclidinone）,achieving higher phase transition temperatures and switchable dielectric and second-order nonlinear optical switching characteristics.This indicates that using the difference in molecular symmetry can not only effectively adjust the phase transition temperature of the crystal,but also adjust the symmetry of the crystal to achieve specific functions.（3）（1-221）₂Co Li（NO₂）₆（1-221 is 1-azabicyclic[2.2.1]heptane）was constructed,and no structural phase transition was found in this compound.Based on（1-221）₂Co Li（NO₂）₆,using R-3HQ（R-3-hydroxylquinuclidinium cation）with lower symmetry,a polar nitrite bridged bimetallic perovskite（R-3HQ）₂Co Li（NO₂）₆（9）was prepared,achieving high temperature ferroelectricity（Tc=339 K,saturation polarization Ps=0.62μC/cm²）.Therefore,reducing the molecular symmetry of organic cations by introducing chirality is an effective means to obtain polar solids and achieve ferroelectric and related functions.These studies indicate that the structural phase transitions of molecular crystals are highly correlated with molecular symmetry characteristics in specific systems.Reducing molecular symmetry can effectively adjust the structural phase transition properties of molecular crystals,achieving or adjusting physical properties such as ferroelectricity and piezoelectric properties.This study provides ideas for further adjusting molecular symmetry to achieve novel or enhanced physical properties in molecular crystals.