Study On The Structural Phase Transitions Of Hydrogen-bonded Organic Crystals At High Pressure And Low Temperature

Hydrogen bonds is one of the most common and representative intermolecular weak interaction.Compared to covalent bonds,hydrogen bonds can be more easily be tuned by compression and temperature conditions in organic systems.It is well known that the physicochemical properties of materials are strongly related to their crystal structures.The study of hydrogen-bonded organic crystals at high pressure and low temperature are essential to exploring the relationship of structure and properties,and hence establishing the basis for function-oriented material design.Firstly,a very typical hydrogen-bonded organic crystal Tris（hydroxymethyl）aminomethane（named THAM hereafter）was selected for low temperature research by using four-circle single crystal diffractometer.As temperature decreases,it was found that the average intensity of hydrogen bonds increases.At 190 K,the hydrogen bonds were distorted and deformed and the related bond length were stretched,resulting in a second-order phase transition.In addition,the a-axis of THAM has obvious negative thermal expansion at low temperature.The mechanism behind is found that the C-O bond in the O₁-C₂-C₁-C₄-O₃ framework which almost paralleled to the a-axis direction abnormally extended at low temperature,resulting in the a-axis has negative thermal expansion property.Secondly,high pressure behaviors of THAM single crystal were carried out in diamond anvil cell（DAC）by four-circle single crystal diffractometer.The measured results show that THAM undergoes a phase transition at 1.53 GPa approximately.Meanwhile,the crystal structure of THAM at high pressure were simulated by first-principles calculations method.Combined with high pressure X-ray experiments,it provides sufficient evidence that the phase transition can be attributed to the formation of new hydrogen bonds and the phase transition is reversible.Furthermore,THAM crystal has large negative linear compression in the c-axis among 1.53².67 GPa.It can be attributed to the formation of hydrogen bonds N₁-H₂...O₂ leaded to stretch of the N₁-C₁-C₃-O₂ framework,during the pressure increases the hydrogen bonds strength enhances which cause the framework continues to stretching.As the framework approximately paralleled to the c-axis,thus results in negative compression of the c-axis of THAM.Finally,p-PHCN-DHA single crystal was selected for high pressure measurements by four-circle single crystal diffractometer and Raman spectrometer.It was found that the p-PHCN-DHA single crystal undergoes a phase transition of at 0.1⁰.31 GPa approximately.The steric hindrance effect in the molecule enhances as pressure increases,resulting in the stretch and deformation of the benzene ring and the C=C bond and the related phase transition.