Raman Study Of Phase Transitions On C₆H₆ And CCI₄ Under High Pressure

With the improving of molecular spectroscopy and the developing of diamond anvil cell technology , the Raman study is widely used in molecular crystal structure under high-pressure, it has a significant contribution in the molecular bond strength between the role of co-existence especially. In recent years , the molecular dynamics have been the subjects of much research interest., which the representative of substances are benzene, carbon tetrachloride and other hydrocarbons. At present, there are not many reports of benzene and carbon tetrachloride molecules under high pressure and the pressure are relatively low. This experiment will increase the pressure to 12.87GPa by the DAC. The situ signal under high pressure was collected by the Raman spectroscopy, Detailed analysis the phase transitions of the benzene and carbon tetrachloride molecules in the pressure range, Study the changes of the molecular structure of the sample under high pressure. Explaining the Fermi resonance of the sample changes with the pressure be increased.Phase transition of benzene has been carried out for many years, the form of benzene has been very clear under high pressure. The Raman spectra of benzene has very significant changes with the increased Pressure, including the peak changes of relative intensity, frequency moving, half-width and the production and disappearance of Fermi resonance, etc, in which the scientific information is very rich, it is necessary to be given more attachment. This experiment which combines previous research results increases the pressure on the liquid benzene by DAC, analysising molecular structure changes of benzene (0-12.89GPa). The first phase transition of benzene occurred When the pressure reached 0.06GPa. At this time there are four molecules per unit cell in benzene molecules, its space group is(D2h15).As other samples the Raman vibrational spectra of benzene move to higher frequency with the increased pressure, mainly because the pressure speed up the frequency of the vibration .Ring breathing vibrationυ1 (987 cm-1) of benzene splits ,when the pressure reached 4.03GPa. And there is a new peak nearυ6 (607 cm-1) at this pressure. At this phase transition point, the benzene molecules transit from II phase to III phase. The intensity ratio ofυ1和υ1D changes, With the pressure increasing .It is almost equal at the 9.89GPa and the Fermi resonance phenomenon disappeared at this point. This is because both breathing modeυ1 (987 cm-1) and benzene ring in-plane deformation vibration modeυ6 (607 cm-1) of benzene occur phase transition under high pressure. There is a new peak appear near the benzene ring plane deformation vibrationυ6 (607 cm-1), when the pressure reaches 10.89GPa, This proves that the second-order phase transition of benzene molecules take place at the 10.89GPa. We get research results that phase transition point of the benzene is 0.06GPa, 4.03GPa, 10.89GPa by the Raman slope curve changes of Pressure - frequency shift. In addition, through the experimental results of measurement and the calculation formula, we get the Fermi resonance value of R f/a,?,ωand ?0 under the pressure. We found that with the pressure increases the spacingΔ0 of the breathing mode of benzeneυ1 (987 cm-1), benzene ring plane deformation vibration modeυ6 (607 cm-1), and benzene ring stretching mode C-Cυ8 (1580 cm-1) is increasing, the coupling constantωof Fermi resonance is constantly decreasing, and R f/a is also decreases. This is mainly due to the difference Raman bands blue shift velocity ofυ1 (ring breathing a1g) +υ6 (ring deformation e2g) combination and theυ8 (ring stretching e2g) fundamental, Lead to the frequency difference increased, the coupling coefficientωand the intensity ratio R f/a decrease.In high-pressure Raman experiments of ccl4 molecules we found that the Raman peak of carbon tetrachloride are blue shift. With increasing pressure the Raman line split. Raman line become narrow at low wave number and lattice patterns create illustrates that the rules of CCl4 are going through a process to no rules crystal. Fermi resonance peaks exchange position and reduced coupling shows that CCl4 have different phase transitions under different pressures.This thesis will provide the experimental basis for the application of liquids such as benzene and carbon tetrachloride molecules in materials science , also provide an effective means to in-depth study the micro-structure of benzene and carbon tetrachloride molecules under pressure .