Symmetry And Physical Properties Of Molecule Ferroelectrics

Molecular ferroelectric crystals are dielectric crystals formed by molecules or molecular ions through van der Waals forces,electrostatic forces.Molecular ferroelectrics are an important functional material in the field of electronic technology.The polar crystal structure can exhibit spontaneous polarization,whose direction can change with the external electric field.This special crystal structure can have non-volatile ferroelectric switching effect,piezoelectricity,pyroelectricity,and non-linear optical effect.From the research progress of molecular ferroelectrics in recent years,it can be seen that the research on molecular ferroelectric phase transition symmetry is maturing.The structural symmetry breaking in the ferroelectric phase change process,the symmetry change of various physical properties(dielectric,piezoelectric,nonlinear optics,pyroelectric,etc.)have gradually become the systematic method for studying molecular ferroelectrics.However,the impact of the symmetry breaking of the ferroelectric phase transition is far more than the evolution of the physical properties mentioned above.With the development of new technologies in recent years,research on the symmetry and physical properties of molecular ferroelectric crystals needs urgently to establish new characterization methods on the basis of new technologies with in situ,real-time,and micro-zones,which have become a frontier field of molecular ferroelectric research.In this dissertation,we designed and synthesized three new types of multifunctional molecular ferroelectric materials.Based on the morphology of materials such as single crystals or thin films,we studied the physical properties allowed by the symmetry requirments of ferroelectricity,such as dielectric,piezoelectric,SHG,pyroelectric,and ferroelectric properties.Moreover,we also introduce additional interesting physical properties into molecular ferroelectric crystals.Through molecular design and crystal engineering,molecular ferroelectric materials can perform many interesting functions.For example,optically active molecular ferroelectrics with optical switch characteristics,molecular ferroelectrics with antiferroelectric characteristics,and molecular ferroelectrics with narrow bandgap semiconductor characteristics.These physical properties have enriched the research scope of molecular ferroelectrics to a certain extent,and provided certain material support and technology accumulation for the combination of molecular ferroelectricity and other research fields.In chapter 2,we successfully constructed a series of optically active multiaxial molecular ferroelectrics by introducing a homochiral organic molecule as a polar component.We have found that the ferroelectricity in the(R)-(-)-3-hydroxyquinuclidine halide series is due to the helical arrangement of homochiral organic molecules.We observe that both the specific optical rotation and rotatory direction change upon paraelectric-ferroelectric phase transitions,due to the existence of two origins from the molecular chirality and spatial arrangement,whose contributions vary upon the transitions.The optical rotation switching effect may find applications in electro-optical elements.In chapter 3,we report that polarizable antiparallel dipole arrays can be realized in an organic-inorganic hybrid perovskite,(3-pyrrolinium)Cd Br3,which not only exhibits an excellent ferroelectric property(with a high spontaneous polarization of 7.0 ?C/cm2),but also presents a striking AFE characteristic revealed by clear double P-E hysteresis loops.To the best of our knowledge,it is the first time that such successive ferroelectric-antiferroelectric-paraelectric phase transitions has been discovered in organic-inorganic perovskites.Besides,a giant dielectric constant of 1600 even at high frequency of 1 MHz and a bulk electrocaloric effect with entropy change of 1.18 J K-1 kg-1 under 7.41 k V/cm are also observed during the phase transition.Apparently,the combined striking AFE characteristic and giant dielectric constant make(3-pyrrolinium)Cd Br3 a promising candidate for next generation high-energy-storage capacitors.In chapter 4,ferroelectric materials have abnormal photovoltaic characteristics.The exploration of photovoltaic materials based on ferroelectric semiconductors has always been a goal pursued by scientific researchers.Among them,the search for narrow band gap ferroelectric semiconductor materials is the primary task of this ambitious project.We designed an organicinorganic hybrid ferroelectric:(2-(ammoniomethyl)pyridinium)Sb I5.It shows an above-room-temperature Curie temperature(Tc=360 K),a large spontaneous polarization(Ps=4 ?C cm-2)and a narrow bandgap(2.03 e V),which is much smaller than the classical ferroelectric semicondtor Bi Fe O3(Eg=2.7 e V).The implementation of ferroelectricity in hybrid semiconducting materials may be a feasible way to realize high-performance ferroelectric optoelectronic and photovoltaic devices.In summary,research work based on the symmetry allowed or specific physical properties of molecular ferroelectric crystals has been carried out in this dissertation.On the basis of studying the symmetry allowed physical properties in molecular ferroelectric crystals,the study of optical rotation,antiferroelectricity and semiconductor properties in molecular ferroelectric crystals are discussed.This paper interprets molecular ferroelectric crystals in more physical dimensions.The relationship between structural symmetry and physical symmetry in molecular ferroelectric systems is studied,providing certain materials and technology accumulation for the development of multi-functional photoelectric materials.