Design And Research Of Molecular Ferroelectrics Based On Highly Symmetrical Amines

Ferroelectrics have been widely applied in ferroelectric memories,detectors and light-emissive devices for the splendid dielectricity,piezoelectricity,ferroelectricity and non-linear optical properties.Conventional inorganic ceramic ferroelectrics BTO and PZT have dominated related fields for decades for the outstanding ferroelectricity and mechanical strength.With the concern of the growing environmental and energy issues,organic-inorganic hybrid ferroelectrics have developed rapidly and become the credible reinforcement of inorganic ferroelectrics.Apart from the construction of polar structures,optimizing ferroelectricity(enhancing TC,increasing number of polar axes and spontaneous polarization)and realizing multifunctionality become remarkable challenges.Through precise molecular modification and crystallographic engineering,three novel molecular ferroelectrics were successfully synthesized and corresponding properties were characterized and investigated.(1)In chapter II,through introduction of reasonable substitutent and H/F substitution,a molecular ferroelectric[FEtdabco]Zn I3 with a record high Curie temperature was synthesized.The multi-axial ferroelectricity was confirmed by measurements of P-E hysteresis loop and PFM,according with the feature of ferroelectric-paraelectric phase transition with Aizu notation of 4/mmm Fmm2.(2)In chapter III,highly efficient photoluminescent inorganic framework Mn B4^2-was utilized to conceive and synthesize a series of hybrid manganese bromide compounds[RQ]2Mn Br4 by introducing precisely designed quasi-spherical cations[RQ]⁺(R=H,Me,Et,FEt,Q=quinuclidine).The accurate and effective modification of cations not only realizes the transition from the central symmetric space group to the polar space group and achieves the satisfactory ferroelectricity(a Curie temperature of 378 K and a saturated polarization of1.8?C/cm²),but also regulates the Mn…Mn distances of Mn Br₄^2-anions and consequently enhances the photoluminescence quantum yield(PLQY)from 38.7%to 83.65%.This work provides an efficient strategy for enriching of organic-inorganic hybrid ferroelectrics with highly efficient light-emissive properties.(4)In chapter IV,the relatively light Cl/Br atoms were replaced by I atoms,the free rotation of crystal was successfully frozen.Such an ordered state enhanced the intermolecular interactions induced by the F atoms.Specifically,the resulting[FEt Q]2Co I4 crystalizes in a polar point group mm2 and undergoes a ferroelectric-paraelectric phase transition with Aizu notation of 4/mmm Fmm2.The step-like dielectric anomalies supply the potential for dielectric switch.The solid state UV-vis spectrum measurement and theoretical calculation of the band structure confirmed[FEt Q]2CoI4 a direct band gap semiconductor with an Eg of 3.25 eV.In summary,this thesis investigated three novel ferroelectrics with outstanding ferroelectricity designed by precise molecular modifications.The modulation of extra physical properties should inspire more composite design methods to control and optimize the properties of organic-inorganic ferroelectric multifunctional materials.