Design,Synthesis And Ferroelectric Properties Of Ferric-based Molecular Materials For Multipolar Axes

Ferroelectric materials are gaining more and more attention due to their applications in information storage,photoelectric detection,diodes,etc.Among them,molecular-based ferroelectric materials are one of the important branches of ferroelectric materials,which are the development trend and application hotspots of ferroelectric devices due to their diverse structures,simple regulation,low cost of film production,and degradability.According to the number of poles,ferroelectrics can be divided into two categories:unipolar-axis ferroelectrics and multi-polar-axis ferroelectrics.Due to the small number of polar axes,it is difficult to control the ferroelectricity in the direction perpendicular to the film when it is made.Compared to unipolar-axis molecular-based ferroelectrics,multipolar-axis molecular ferroelectrics have their own unique advantages in thin-film devices due to their unique multipolar-axis nature.Therefore,it is important to design and obtain molecular-based ferroelectrics with multipolar axis properties by chemical synthesis.The main study of this thesis is the design and synthesis of organic-inorganic hybrid molecular-based multipole-axis ferroelectrics based on iron-based halides and their ferroelectricity.The main research of this thesis is as follows:（1）Two examples of organic-inorganic hybrid compounds[TMTP]Fe Br₄（1）and[TMTB]Fe Br₄（2）were synthesized by using tetramethylamine derivatives trimethyl-N-isopropylamine（TMTP）and trimethyl-N-tert-butylamine（TMTB）to react with Fe Br₃,respectively.X-ray single-crystal diffraction of indicated that compounds 1 and 2 both crystallized in the centrosymmetric space group.DSC curves tested showed that compounds 1 and 2 undergo phase transitions at 413 K and 470 K,respectively.And their N（N indicates the degree of molecular disorder）was calculated to be 9.03 and 4.72,respectively,indicating that compounds 1 and 2undergo phase transitions belonging to the ordered-disordered type.The dielectric curves at 1 MHz indicate that dielectric anomalies are observed at 415 K and 442 K for compounds 1 and 2,respectively,due to the transition of the organic amine cations in compounds 1 and 2 from the ordered state at low temperature to the disordered state at high temperature.（2）The iron salts were reacted with quinuclidine（Q）and its derivatives quinuclidinone（3-O-Q）and quinuclidinol（R3HQ）,respectively,to give three examples of compounds[Q]Fe Br₄（3）,[3-O-Q]Fe Br₄（4）and[R3HQ]Fe Br₄（5）.Compounds 4 and 5 both crystallize in the polar point group.The DSC curves and dielectric curves of compounds 3-5 observe phase transition in the same temperature range,which indicates that compounds 3-5 have a reversible photoelectric switching response.P-E curves test the surface compounds as ferroelectrics and are inferred to have 2 electrode axes based on Aizu’s theorem.（3）To further increase the phase transition temperature and the number of electrode axes of compound,the low symmetry N-methyl-quinuclidine（N-CH₃-Q）and N-methyl-quinuclidine alcohol（RM3HQ）were used to react with Fe Br₃ to obtain compounds[N-CH₃-Q]Fe Br₄（6）and[RM3HQ]Fe Br₄（7）,respectively.DSC and dielectric tests demonstrated that the phase transition temperatures of compounds 6 and 7 were higher than room temperature.Both compounds 6 and 7crystallize in the polar point group,and it was demonstrated by Aizu’s theorem and PFM tests that compound 7 undergoes a 2F1 ferroelectric phase transition and a m3m F1 ferroelectric phase transition with up to 12 ferroelectric axes during the ferroelectric phase transition.This work achieves the goal of high temperature Curie temperature and multipole axis of molecular ferroelectricity and contributes to the thin film devices of molecular ferroelectric materials.