Syntheses,Structures And Properties Of Phase Transition Compounds Based On Di-n-butylamine

Phase transition materials have attracted great attentions owing to the unique thermal,optical,electrical,mechanical and magnetic properties shown during the phase transition.In order to explore new phase transition crystal materials,we have synthesized a series of phase transition compound.Furthermore,differential scanning calorimetry,specific heat,variable-temperature dielectric,variable-temperature second-harmonic generation,pyroelectric,variable-temperature X-ray structure determinations were used to investigate deeply their mechanism and physical properties of phase transition.1.Di-n-butylamine-based molecular compounds are found to readily undergo phase transition,hence we use di-n-butylamine as the functional unit,fumaric acid as the anion and synthesized the phase transition compound[(n-C4H9)2NH2]2H2C4O4·H4C4O4(1).Thermal measurements reveal that 1 undergoes a reversible ferroelastic phase transition at 228.8 K.The structures of high temperature and low temperature have been measured and analysized to further study the phase transition mechanism.Variable-temperature single-crystal X-ray diffraction analyses reveal that the cooperative displacements of hydrogen bonds induce the structural phase transition,which arise from the twisting motions of the fumaric acid molecules.Simultaneously,two types of independent hydrogen bonding layers in the entanglement are altered in response to the transformation of hydrogen bonds aggregates at low temperature phase,causing the symmetry breaking.2.Di-n-butylamine-based molecular compounds are found to readily undergo phase transition due to the twisting transformation of dibutyl configuration,thus we use di-n-butylamine as the functional unit,trifluoroacetic acid as the anion and synthesized the phase transition compound di-n-butylaminium trifluoroacetate(2),which shows superior pyroelectric performance.Thermal measurements reveal that 2 undergoes a reversible first-order ferroelectric phase transition at 212 K.Its pyroelectric figures of merit are found to exhibit a remarkable enhancement as transition temperature approaches,which are almost an order of magnitude larger than those of other materials,such as TGS and inorganic PMNT.Moreover,it possesses the highest M1 value of 56×10-3 Cm2/?C among the known molecular pyroelectrics,revealing its potentials to obtain highly-efficient signal-to-noise ratio.Further structure analyses reveal that reorientation of the molecular dipoles generates the molecular polarities,that is,the potential ferroelectric order induced by a synergetic ordering of cations and anions.Pyroelectricity exists in the polar dielectrics with spontaneous polarization.3.Di-n-butylamine-based molecular compounds are found to easily undergo phase transition due to the twisting transformation of dibutyl configuration,thus we use di-n-butylamine as the functional unit,dichloroacetic acid as the anion and synthesized the phase transition compound di-n-butylaminium dichloroacetate(3).Thermal measurements reveal that 3 undergoes a reversible first-order phase transition at 225.0 K.Interestingly,the subnanometre-scale changes during phase transition are instantly amplified to a micrometre-scale contraction or expansion of the crystal.Furthermore,the shape change is anisotropic.Namely,the thermal expansion along the c axis is as large as 4.08%close to the phase transition temperature,but a negative thermal expansion(NTE)occurs along the b axis.The structures of high temperature and low temperature have been measured and analysized to further study the mechanism of shape change induced by phase transition.During the phase transition,the di-n-butylaminium cation becoming ordered causes the configurational torsion of terminal moiety,which results in the reorientation of cation.Eventually,the change in the orientation of the di-n-butylaminium cation induces a huge macroscopic deformation of the crystals along c axis.