Dielectric And Ferroelectric Properties Of Molecule-based Phase Transition Bistable Crystalline Materials

Molecule-based phase transition bistable crystalline materials are a class of novel materials that can undergo dielectric,piezoelectric,pyroelectric,ferroelectric and second-harmonic nonlinear optical responses upon external stimulus,which have an extensive range of technological utilizations in ferroelectric nonvolatile memories,piezoelectric sensors,pyroelectric detectors,optical modulators,electrical-optical switches,and laser frequency multipliers,etc.In the light of Neumann'principle and Curie'principle,electrical and optical properties of crystals are inseparable from crystal structure and symmetry.For example,ferroelectricity exists only in the specific 10 crystallographic polar point groups,that is,1?C₁?,2?C₂?,m(C_1h),mm2(C_2v),4?C₄?,4mm(C_4v),3?C₃?,3m(C_3v),6?C₆?,and 6mm(C_6v).Notably,phase transition of crystalline compounds often leads to changes in crystal structures and symmetries caused by molecular motion.Moreover,affected by such microscopic molecular motion,relevant macroscopic physical properties will also exhibit obvious anomalies in the vicinity of the phase transition point.Especially for ferroelectric phase transitions from a paraelectric phase to a ferroelectric phase,the crystal structure will undergo symmetry breaking together with the occurrence of reversible spontaneous polarization,at the same time,a significant temperature-dependent dielectric anomaly will appear near the Curie temperature?T_c?.Therefore,in order to obtain more novel molecule-based ferroelectrics,simple and convenient dielectric measurements could be used to screen out structural phase transition compounds firstly,and then their ferroelectric properties should be further verified,which turns to be an effective method to explore new molecule-based ferroelectric materials.Based on molecular design and crystal engineering,we have found several simple and flexible ammonium cationic moieties,such as ammonium?[NH₄]⁺?,methylammonium?[MeNH₃]⁺,Me=-CH₃?,1,4-diazabicyclo[2.2.2]octylammonium([H₂dabco]²⁺),diisopropylammonium?[?Me₂CH?₂NH₂]⁺?,pyridinium,imidazolium,quinuclidium,etc,which are particularly suitable for construction of molecule-based phase transition dielectrics and ferroelectrics.These small molecular moieties are prone to experience motions or rotations triggered by external temperature and thus contribute to structural phase transitions.Similarly,it is also possible to select small anionic moieties that can easily move or rotate,including tetrahedral[ClO₄]^-,[BF₄]^-and[XCrO₃]^-?X=F^-?Cl^-?Br^-?anions.Thereby,the carefully selected and designed small molecular ammonium cations and appropriate inorganic anionic building blocks are assembled into crystal compounds of different dimensions,which can possess good structural tunability,thus realizing dielectric bistable switching of phase transition materials and multifunctionality of electrical and optical switches.Inspired by above-mentioned analyses,three novel molecule-based phase transition bistable dielectric and ferroelectric compounds could have been designed and synthesized.Herein,their crystal structures,phase transition behaviors,relevant physical properties such as dielectricity,piezoelectricity,SHG effects,pyroelectricity and ferroelectricity,etc are studied and discussed detailedly.And the conditions and rules for diverse functional properties are summarized,which lays a foundation for future realization of the controllable synthesis of multifunctional phase transition materials.?I?In Chapter Two,a unique molecular design strategy is proposed to reduce the symmetry of dabco molecule by adding a methyl-CH₃,and then employing a larger RbI₃ framework.As expected,an organic-inorganic hybrid multiaxial ferroelectric[MeHdabco][RbI₃]?1?was successfully obtained,which crystallizes in the low-symmetric polar point group C₃ and undergoes a ferroelectric phase transition with high-T_c of 430 K.The excellent multiaxial ferroelectric nature of 1 has been demonstrated by piezoresponse force microscopy?PFM?and polarization-electric field?P-E?hysteresis loop.?II?In Chapter Three,based on flexible small molecular cation[C₃H₅N₂]⁺and tetrahedral ionic moiety[FCrO₃]^-,a reversible phase transition compound imidazolium flurochromate?[C₃H₅N₂][FCrO₃]??2?was designed,which exhibits switchable optical and electrical properties with T_tr of 233 K.And large bulk single crystal of 2 was successfully fabricated at ambient conditions.?III?In Chapter Four,a molecular switching crystal compound[?Me₂CH?₂NH₂]₂[CdCl₄]?DPAC,3?with optical/electrical/thermal/mechanical multiple bistable states was designed.And the large-size single crystal of 3 with high-T_tr of 440.7 K was successfully prepared.