Phase Transition Of Two Typical Hydrogen-bonded Organic Crystals Under High Pressure

Pressure is a basic thermodynamic parameter.It can cause a series of changes in properties such as the distance between atoms,the arrangement of atoms,the crystal structure,and the structure of electron orbits.For organic crystals,pressure can change the molecular spacing,the structure of the crystal and the symmetry of the molecular point group,which in turn induces a phase transition in the crystal structure.This has important scientific significance for studying the structure and properties of organic crystals.There are a large number of hydrogen bonds in organic crystals.And the bond energy of hydrogen bonds is lower than that of covalent bonds.The hydrogen bonds in organic crystals are often more susceptible to pressure and lead to phase transition in the crystal structure of matter（such as the formation of hydrogen bonds,rearrangement,fracture and symmetry）.The change of crystal structure may lead to phase transition and affect its physical and chemical properties.After the phase transition of matter,new physical phenomena or new matter may appear.Therefore,the phase transition of hydrogen bonded organic crystals under high pressure can provide useful help for scientific research in the structural change of substances and other aspects.It can let us know more about hydrogen bond,and bring more help and understanding to the synergistic effect of organic crystal structure,properties and intermolecular interactions.The synergy effect brings more help and understanding.In this paper,two hydrogen bond organic crystals of ascorbic acid and urea peroxide are selected.We used diamond anvil cell,Raman spectroscopy technology and synchrotron radiation X-ray diffraction technology to study the changes of these two crystals under high pressure and the structural phase transition.Ascorbic acid（C₆H₈O₆）,also known as vitamin C,is an acidic and strongly reducing polyol with six carbon atoms.In this work,ascorbic acid was studied by in-situ high pressure Raman spectroscopy.Through the analysis of the results,we found that most of the Raman peaks move to higher wavenumbers as the pressure increases.Ascorbic acid undergoes a phase transition at a pressure of about 3.8GPa.And it is found that some peaks have obvious frequency shift and intensity changes during phase transition(such as 583,693 and 1746 cm^-1).We believe that the structure of the inner and side chains of the ascorbic acid ring has changed under high pressure and caused a decrease in the crystal symmetry structure.In addition,due to the existence of hydrogen bonds in ascorbic acid crystals,we also think that this may also be related to the breaking and rearrangement of the hydrogen bond structure.Through the analysis of ascorbic acid Raman spectroscopy,we found that the observed transition was completely reversible when the system was brought back to ambient pressure.Urea hydrogen peroxide（CO（NH₂）₂·H₂O₂）is a crystalline adduct of hydrogen peroxide and urea.It has a wide range of uses in daily life and production,and has broad application prospects as a new type of fine chemical product.In this paper,urea hydrogen peroxide was studied by in-situ high pressure Raman spectroscopy and synchrotron radiation X-ray diffraction.By analyzing the high-pressure Raman spectrum of urea hydrogen peroxide,we believe that the phase transition occurs at a pressure of about 2.5GPa.And through the measurement of the sample after depressurization,it was found that the observed transition became irreversible.Finally,in order to confirm the pressure-induced phase transition and understand the structural changes under high pressure,we conducted ADXRD experiments on urea hydrogen peroxide.We also used the software Materials Studio to refine and simulate the phase transition urea hydrogen peroxide based on the experimental results,and obtain its possible lattice constant.