Design And Performance Tuning Of Multifunctional Molecular Ferroelastic/Ferroelectric Crystals

Ferroelastic and ferroelectric crystals with respective switchable spontaneous strain（S_s）and spontaneous polarization（P_s）under an external field（stress or electric field）have been widely used in the high-tech fields such as sensors,actuators,energy conversion,and information storage.Their phase transitions must fall into the 94 ferroelastic or 88 ferroelectric species defined by Aizu,making it a challenging task to design and synthesize ferroelastic/ferroelectric materials.Obviously,the symmetry requirements for the coexistence of the two properties are more stringent.While inorganic oxides have long occupied the mainstream of research and application for decades,organic and organic-inorganic hybrid molecular materials with the advantages of lightweight,environmental friendliness and energy saving,flexibility,easy processing and easy film formation are expected to be their feasible supplements as well as a bridge to the future soft composite materials.Particularly,the inherent chemical diversity and structural tunability of molecular systems also render the great benefit to the fine-tuning of ferroelastic/ferroelectric performances and the introduction of multiple functions.Therefore,molecule-based ferroelastic/ferroelectric materials become a hot topic in materials science.However,to realize their practical usefulness and potential in the next-generation flexible,wearable,and multifunctional electronic devices,the key obstacle to be overcome is the limited number of existing material systems with excellent performance（such as high Curie temperature T_C,multiaxial feature,large P_s and S_s,etc.）.In this sense,it is imperative to deepen the understanding of the structure-property relationship by exploring various effective methods for the structural design,performance regulation,and multi-functionalization of molecular ferroelastic/ferroelectric materials.In the context of molecular design and crystal engineering,selecting flexible organic cations that easily undergo motion or rotation from the point view of"quasi-spherical"theory and then combining strategies of halogenation effect,atomic substitution,introduction of homochirality and solid solution,herein a series of ferroelastic and ferroelectric molecular crystals were synthesized,and their phase transitions,structures and dielectric,nonlinear optical effect,pyroelectric,ferroelectric,ferroelastic and other functionalities were analyzed and discussed in detail.（I）The halogenation effect modulates ferroelasticity and regulates the phase transition temperature.In Chapter Two,based on the quasi-spherical dimethyl-isopropyl-ethyl-ammonium cation（[DMIE]⁺）,an organic-inorganic hybrid perovskite[DMIE][Cd（SCN）₃]（1）was designed and synthesized.Compound 1 undergoes an ordinary structural phase transition from the space group P6₃ to P6₃/mmm.After modifying the[DMIE]⁺cation with halogen atoms F,Cl,and Br,respectively,the symmetry groups of the corresponding perovskite materials 2-4get smoothly lowered to Pbca to induce the ferroelastic phase transitions with an Aizu notation of 6/mmm Fmmm.As the halogen atomic number increases,the spontaneous strain value reduces from 0.0778 to 0.0428,whereas the phase transition temperature increases from 248.6to 367.8 K.（II）Molecular solid solution is beneficial to the optimized ferroelastic properties and the emergingnonlinear optical property.In Chapter Three,based on the quasi-spherical halogenated quaternary phosphonium cations,a molecular ferroelastic perovskite solid solution[（EPCF）_x（EPCCl）_1-x][Mn（dca）₃]（EPCF=[Et₃P（CH₂）₂F]⁺,EPCCl=[Et₃P（CH₂）₂Cl]⁺,and dca=dicyanamide,0≤x≤1）was designed and synthesized.When x=0.170 to 0.405,unexpectedly,the mixing of two centrosymmetric prototypical phases gives rise to a chiral P2₁2₁2₁ phase with second harmonic generation（SHG）response.Besides,it also brings forth the improvement of ferroelastic performance and the temperature of long-range magnetic ordering.（III）The flexible cyclic organic molecule plays a role in constructing polar multifunctional materials.In Chapter Four,by replacing the conjugated thiazole cation with the more flexible thiazolidine,an organic-inorganic hybrid perovskite material[（CH₂）₃NH₂S][Cd Br₃]（5,[（CH₂）₃NH₂S]⁺is thiazolidinium cation）was designed and synthesized.Its phase transition temperature has been greatly increased to 263 K,and the crystal structure has been successfully converted from the original centrosymmetry to be polar.The resultant SHG,piezo-/pyroelectric effects may find applications in self-powered low-voltage electronic devices and energy harvesters.（IV）Molecular modification introduces multiaxial ferroelectricity and modulated phase transition behavior.In Chapter Five,a supramolecular ferroelectric[（ATHTP）（18-crown-6）][BF₄]（6,where[ATHTP]⁺is protonated 4-aminotetrahydrothiopyran）was synthesized by replacing the para position C atom of the cyclohexylammonium cation[Hcha]⁺in the uniaxial ferroelectric[（Hcha）（18-crown-6）][BF₄]with the S atom.While retaining ferroelectricity,it has also been successfully endowed with multiaxial feature and sequential structural phase transitions.The T_C of up to 392 K is prominent in other crown-ether-based inclusion complex ferroelectrics.（IV）The introduction of bimetal centers and homochirality enables the coexistence of multiple functional properties such as multiaxial ferroelectricity,ferroelasticity and so on.In Chapter Six,the hybrid bimetal halide multiaxial ferroelectrics（3-hydroxypyrrolidinium）₂Rb Bi Br₆（7）and（R-3-hydroxypyrrolidinium）₂Rb Bi Br₆（8）were designed and synthesized.The inorganic framework adopts a rare 4-connected lon topology.And they both exhibit an exceptionally high Curie temperature（432 and 485 K,respectively）,a relatively large spontaneous polarization(5.73 and3.18μC cm^-2,respectively).Besides,the homochiral ferroelectric 8 has diverse functionalities including ferroelasticity,circularly polarized luminescence,semiconducting characteristic,etc.With a view to promoting the application development of molecular ferroic materials,in this thesis,the systematic study for the controllable synthesis and performance optimization of novel multifunctional molecular ferroelastic/ferroelectric materialss was conducted by exploring the introduction of ferroelasticity,the design of polar structures and the modulation of ferroelectricity.