Study On Design And Regulation Of Dielectric Phase Transition Characteristics Of Crystalline Quaternary Ammonium Salts

Smart response and information processing have been an important issue related to the living and daily production of human being.Stimulus responsive materials that can make corresponding signal actions in response to external stimuli have recently attracted much attention,especially dielectric switch and nonlinear optical effects.However,it remains a large challenge to design such stimulus responsive materials,especially functional materials having both dielectric switch and second harmonic generation(SHG).In this thesis,4 novel organic-inorganic hybrid compounds were synthesized and studied:[(CH₃)₃N(CH₂)₂Cl]₂[Mn(SCN)₄(H₂O)₂](1),[(CH₃)₃N(CH₂)₂Cl][Cd(SCN)₃](2),[C₈H₁₅N₂][Cd(SCN)₃](3),[C₉H₂₀N][Cd(SCN)₃](4).Through single crystal structure data analysis,dielectric property analysis,differential scanning calorimetry(DSC)and variable temperature powder X-ray diffraction measurement(PXRD)to further determine whether these compounds have undergone reversible phase transition.It was found that Compound 1,Compound 2 and Compound 3 are novel stimuli-responsive switchable materials.It is worth noting that 1 has SHG characteristics before and after undergoing reversible high-temperature phase transitions.Compound 2 has SHG characteristics before the reversible high-temperature phase transition,but the SHG signal disappears when the phase transition occurs.It shows that 2 was a potential material with rare dual-switch response characteristic.We successfully refined the tetramethylammonium cation to obtain a quasi-spherical cation,that is chloroethyltrimethylammonium cation.By substituting H with a halogen,the increased steric hindrance of the molecular makes energy barrier increased,resulting in the reversible high-temperature phase transition.At the same time,the interactions of quasi-spherical cations and anions affect a non-centrosymmetric structure to induce the SHG effect.These findings provide a new approach to design novel functional switch materials.