Optical/Electrical Switching And Ferroic Phase Transition Materials Based On Quaternary Phosphonium Cations

Phase transition materials have been used for many years in commercial applications,and their use in energy storage has effectively mitigated fossil fuel reserves and greenhouse gas emissions,increasing energy storage and use efficiency in many household and industrial sectors.Besides,phase transition materials are used in a wide range of applications in the biomedical,electronics,textile,construction and automotive industries.With the development of phase transition materials,many types of phase transition materials have emerged,including inorganic compounds,organic compounds,polymers,and organic-inorganic hybrid materials.Among them,molecular-based phase transition materials（including organic compounds and organic-inorganic hybrid materials）have been extensively studied in recent years due to their advantages of low cost,light weight,easy processing,flexibility and environmental friendliness.Most of the molecular phase transition materials contain small molecular ammonium.It is found that ammonium cations are prone to undergoing order-disorder transitions with changing temperature to induce structural phase transitions.Phosphorus and nitrogen belong to the same main group elements.The phosphonium cations have similar structures with ammonium cations,and they are also easy to induce phase transition under thermal stimulus.In this dissertation,different quaternary phosphonium cations were used to construct novel molecular-based phase transition compounds.Based on the above analysis,several phase compounds containing quaternary phosphonium cations were synthesized and their crystal structures and phase transition properties were characterized by differential scanning calorimetry（DSC）,X-ray single crystal diffraction,and dielectric measurement.（I）In Chapter Two,a series of quaternary phosphonium picrates were designed and synthesized:[Me₄P][PA]（1）,[Me₃（n-Pr）P][PA]（2）,[Me₃（CH₃OCH₂）P][PA]（3）,[Me₃（CH₂=CHCH₂）P][PA]（4）and[Me₃（CH₂CH₂OH）P][PA]（5）（where[Me₄P]⁺is tetramethylphosphonium cation,[Me₃（n-Pr）P]⁺is a trimethylpropylphosphoniumcation,[Me₃（CH₃OCH2）P]⁺isa trimethylmethoxymethylphosphoniumcation,[Me₃（CH₂=CHCH₂）P]⁺isa trimethylallylphosphonium cation,[Me₃（CH₂CH₂OH）P]⁺is a trimethyl（2-hydroxyethyl）phosphonium cation,[PA]^-is picrate,and picric acid is 2,4,6-trinitrophenol）.Compound 1undergoes a phase transition near room temperature.By reducing the symmetry of[Me₄P]⁺,i.e.replacing one of the methyl groups with longer side chains,four compounds 2-5 with high-temperature phase transition were successfully obtained.They exhibit switchable dielectric behaviours near phase transition temperature,and compound 5 also exhibits second harmonic generation（SHG）switching behavior.Different side chains have a noteworthy effect on the structure and properties of the compound.（II）In Chapter Three,three organic-inorganic hybrid phase transition compounds were designed and synthesized based on metal dicyanamide framework,namely,[Et₃（n-Pr）P][Cd（dca）₃]（6）,[Et₃（n-Pr）P][Mn（dca）₃]（7）and[Et₃（CH₂=CHCH₂）P][Mn（dca）₃]（8）（where dca is dicyanamide N（CN）₂^-,[Et₃（n-Pr）P]⁺is a triethylpropylphosphonium cation,[Et₃（CH₂=CHCH₂）P]⁺is a triethylallylphosphonium cation）.Those compounds have a three-dimensional perovskite-type structure,and the quaternary phosphonium cations occupy the voids in the metal dicyanamide framework.Comparison of the three compounds revealed that changes in metal ions or phosphonium cations caused changes in phase transition properties or crystal symmetry,affecting the dielectric properties and the response and switching of SHG effects.（III）In Chapter Four,two ferroic compounds were designed and synthesized:[Me₄P]Cd Cl₃（9）and[Me₃Cp P][Cd（SCN）₃]（10）（where[Me₃Cp P]⁺is trimethylcyclopentylphosphonium cation）.They have a one-dimensional perovskite-like structure,and the phosphonium cations occupy triangular cavities between the anion chains.Compound 9 is a molecular ferroelectric possessing a high-temperature ferroelectric phase transition.Its ferroelectricity is confirmed by the measurement of P–E hysteresis loop,observation of ferroelectric domains,andλ-type dielectric change near the Curie temperature.The Curie temperature,remanent polarization,and dielectric responce of compound 9 were significantly improved compared to the low temperature ferroelectric[（CH₃）₄N]Cd Br₃.Furthermore,compound 9 has Sb³⁺-doped luminescence,which is rare in molecular ferroelectrics.Compound 10 undergoes two ferroelastic phase transition above room temperature and displays switchable dielectric property near the Curie temperature.The ferroelasticity of compound 10 was confirmed by the observed ferroelastic domains and their evolution with changing temperature.The spontaneous strains of two ferroelastic phase were estimated by using the cell parameters of compound 10 in different phases.