| Phase transition materials are becoming the extensive research subject due to their potential applications in data communication,signal processing,storage,etc.Phase transition is accompanied with anomalies of physical properties.In order to find new phase transition materials,we synthesized a series of compounds containing triethylphosphine derivatives.This thesis contains 8 conpounds:[Et3P?CH2?2F]2[CdCl4]?1?,[Et3P?CH2?2F][C6H3N3O7]?2?,[Et3P?CH2?2F][Cd?SCN?3]?3?,[Et3P?CH2?2Cl][Cd?SCN?3]?4?,[Et3P?CH2?2F][Mn?dca?3]?5?,[Et3P?CH2?2Cl][Mn?dca?3]?6?,[Et3P?CH2?2Cl][Cd?dca?3]?7?and[Et3P?CH2?2F][Cd?dca?3]?8?.Phase transition materials have been studied by DSC measurements,dielectric analysis and single crystal structure.Considering the DSC results,there is a pair of endothermic and exothermic peaks,which is consistent with the temperature of the dielectric in compound 1.For 2,two reversible heat anomalies at 348/364 K upon cooling/heating runs were observed.The sharp peaks and large thermal hysteresis around 364 K indicate the first-order types of this phase transition.The dielectric anomalie appeares at about 367 K,in accordance with the DSC result.In compound 2,the reason for phase transition may be the disordered high temperature movement of picric acid and phosphinate groups.There is a pair of heat anomalies on cooling and heating,indicating that compounds 3 and 4 exhibit structural phase transitions.The dielectric anomalie appeare at about 373 K and 343 K in compound 3 and 4,respectly,which is consistent with the DSC results.Although the structures after the phase transition of the compounds 3 and 4 are not obtained,a sharp reduction of the number of diffraction peaks demonstrates occurrence of the phase transitions from low symmetry to high symmetry.There are two pairs of heat anomalies on cooling and heating,indicating that compound 5 and 6 exhibit structural phase transitions in DSC analysis.The dielectric anomalies appeare in compound 5and 6,respectly,which is consistent with the DSC results.According structural analysis,the phase transitions are thought to be the sway of the[Cd?N?CN?2?]-framework and the rotation of guests.Accompanying the sequential phase transitions,they display a broad range of intriguing physical properties,including the above-room-temperature ferroelastic behavior,switchable dielectricity,and low-temperature antiferromagnetic ordering?Tc=2.4 K for both 5 and 6?.These two compounds represent a new class of perovskite-type multiferroic materials that provide an effective means of exploring multiferroic materials.According structural analysis,the point group changes from 222 to mmm to 4/mmm in 7,While the point group changes from 2/m to mmm to 4/mmm for 8 with the increase of temperature.In the view of the fact that noncenter material is second-harmonic generation?SHG?active and 7 undergoes two-step phase transitions from the noncentrosymmetric space group?P212121?to the centrosymmetric space group?Pbcn and I41/acd?,the second-harmonic generation?SHG?measurement is performed at different temperatures to investigate the switch of SHG activities.A step-like change of SHG signals is observed in the 290 K-390 K range,confirming the feature of 7 as a NLO-switching material.After Cl was substituted by F,no SHG response is discovered from 273 K to 460 K for 8,revealing its centrosymmetric structure within this temperature range.Nevertheless,we noticed that the phase transitions from?2 to?2 to?2 phase belong to the 94 species of ferroelastic phase transitions with an Aizu notation of 4/mmmFmmm and mmmF2/m,suggesting it maybe a potential ferroelastic material.Compared with compound 7 and 8,halogen replacement affects not only molecular structures but also physical properties. |
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