Raman Study Of Phase Transitions And Fermi Resonance On CCl₄ And C₆H₆ Under High Pressure

The structural phase transition,chemical reaction and new structure forming induced by pressure have been the subjects of much research interest.As a primary parameter of thermodynamics,pressure has relationship with the total energy of the compressed substance,and the phase structure and chemical reaction Path may be controlled by changing pressure because the interatomic distance can be reduced during compression,which results in the overlapping of adjacent electrons orbit increased.So the pressure dimension can also alter the properties and structure of all matter.Chlorine hydrocarbon is a sort of organic compound which bears biology decompounding and has high poisonous,and Carbon tetrachloride(CCl₄) is one of simple chlorine hydrocarbons.As a typical halogenated hydrocarbon,CCl₄ is an important row material,which is widely applied in the production for industrial chemicals and polymers.Moreover,CCl₄ is also a volatile compound with several important technological applications,being,for instance,used as an etching gas in microelectronics.With rapidly developing of much probe technology,diamond anvil cells(DAC) was widely applied to the high pressure study.The vibration spectroscopy,for example,Raman spectroscopy and IR spectroscopy,is very sensitive in situ technology for investigating the structural transition for the substance in DAC.Under high pressure,some studies on the phase transformation of organic molecule,for example,benzene,have been reported.High-pressure Raman measurement is very useful for the study of molecular crystals which are characterized by the coexistence of strong intermolecular and weak intermolecular bonding.In recent years there has been a growing interest in high-pressure study on the molecular dynamics and phase transitions of methanes,and halogen-substituted methane.As one of simple chlorine hydrocarbons,CCl₄ is simple molecule and is suitable typical molecular crystal.The CCl₄ molecule is highly symmetrical,in the ground state,belonging to the Td point group.There have been very few previous studies of the high-pressure Raman scattering on phase transitions of CCl₄ below the pressure of 3GPa.CCl₄ is a liquid at room temperature and crystallizes at about 230.3 K under atmospheric pressure.On the other hand,four phases have been reported for solid CCl₄ below the pressure of 3GPa at room temperature.Fermi resonance is a very common phenomenon in molecule vibration spectra,especially in polyatomic molecule with complex structure. There have been very few previous studies on molecule Fermi resonance induced by high pressure,but only concerning tri-atomic CO₂ and CS₂.The study of Fermi resonance on the multi-atomic molecular is few,and there is only paper on benzene molecule Fermi resonance reported,but its pressure is very low,only below 2kbar, and the theoretical interpretation of experimental results is very Inadequate.In binary solution,the density and compressibility of the each liquid is different due to the difference in the molecular structure of the each liquid,so the Raman spectra of frequency shift in the binary solution is different from the pure liquid.By comparing the high-pressure Raman spectra of the binary solution and pure liquid,the frequency shift speed change can be detected,therefore the interaction between molecules under high pressure,solvent effects,etc..may be understood.In this thesis,we have investigated the room temperature Raman spectroscopy of CCl₄ and C₆H₆ as well as Binary solution of CCl₄ and C₆H₆ both in compression and decompression from ambient pressure to 13GPa.By analyzing the shifting and splitting of Raman peaks and the changing of Fermi resonance modes,the phase transformation and properties of CCl₄ and C₆H₆,as well as Binary solution of CCl₄ and C₆H₆ at various pressures were obtained.To the best of our knowledge,this is the first report of high-pressure in situ Raman measurements of CCl₄ and C₆H₆ at above 6GPa.CCl₄ and C₆H₆ were purchased from SIGMA Co.Ltd,spectral purity grade.The high-pressure cell used in this experiment is based on the Mao-Bell diamond anvil cell (DAC) having two diamonds with 500μm culet size.The liquid samples with some small ruby chips(～10μm) was loaded in a 200μm hole drilled in a 250μm thick T301 gasket pre-indented to 80μm thickness.Then the cell is carefully pressurized with small steps and allowed to stabilize for a period of time after each pressure change before Raman spectra were taken.The pressure calibration was done using the R₁ and R₂ components of the Ruby fluorescence line.The Raman spectra were recorded using Jobin Yvon LabRAM HR800 and Renishaw inVia Raman Microscopic instrument.Laser excitation at 514.5nm was obtained with a Spectra Physics 160M argon ion laser.The laser power was 6 mw and the typical accumulation time for each spectrum was 30 S.Experiments were conducted up to 13 GPa and were reproduced several times.All measurements were conducted at room temperature.1.Pressure effect on Raman shifting of CCl₄The Raman spectra of CCl₄ at ambient and selected high pressure have been measured at room temperature.The low-frequency modes below 120 cm^-1 are unable to be observed because of the edge filter.In the Raman spectrum of CCl₄ at ambient pressure,there are five Raman peaks,which are located 218,314,459,762 and 790cm^-1.The Raman peaks of 218,314 and 459cm^-1 are assigned to C-Cl symmetric stretching mode v₂(E),C-Cl asymmetric bending mode v₄(F₂) and C-Cl symmetric stretching mode v₁(A₁),respectively.The pair of peaks in the vicinity of 762 and 790cm^-1 is the Fermi resonance doublet,which is due to anharmonic mixing of the v₃ fundamental and the v₁+v₄ combination.Raman spectroscopy is a powerful tool for investigating the conformations and dynamics of molecules in the condensed phase,in particular,for in situ probing phase transitions under high pressure.In earlier papers,CCl₄ was studied up to about 3 GPa by Raman spectroscopy and several solid phases had been reported under high pressure by the internal modes and the librational lattice modes(20-125cm^-1).As pressure increases up to 0.73GPa,the spectral profiles,bandwidths,and relative intensities are almost unchanged.The peak positions of the v₄(F₂) and v₁(A₁) are also unchanged,but the v₂(E) and the pair of Fermi resonance modes gradually shift toward higher frequency on increasing the pressure.The v₂(E) show positive pressure dependence at rate 12.8cm^-1GPa^-1.Ebisuzaki reported that the internal modes(v₂,v₄ and v₁) are unchanged,and the librational lattice modes (20-100cm^-1) occur to change at 0-0.7Gpa.Moreover,a broad Raman peak appeared between v₂ and v₄ at the range of 0.3-0.7GPa.In our experiment,the low-frequency modes below 120 cm^-1 are unable to be observed,and No new peak is detected at the range of 150-900 cm^-1 below 0.7GPa.In particular,CCl₄ in the sample chamber is still transparent.Reference to the previous literature,we infer that the possible phase transitions of the liquid-CCl₄â… ,the CCl₄â… -CCl₄â…¡and the CCl₄â…¡-CCl₄â…¢occur below 0.7GPa.The room Raman data for CCl₄ at the pressures of 0.73 to 7.13GPa show that with increasing pressure,the vibrational frequencies of the internal modes(v₂,v₄ and v₁) and the Fermi resonance modes move to the higher region.The frequency-pressure dependency relationship of the v₂,v₄ and v₁ are linear,and the slopes(dω/dP) are greater than that in the low-pressure region of below 0.73GPa. A new mode appears at a lower frequency(225cm^-1) at 3.03 GPa.As we all know,the CCl₄ molecule has nine normal vibrations,and the lowest frequency is 314cm^-1. Normally,vibrational frequencies move to the higher region with the increasing applied pressure,consistent with that the bonds become stiffened by pressure.So this new peak does not belong to the normal vibration mode of the CCl₄ molecule. Observing the sample chamber,it can be found that the transparent sample transforms into opaque at above 3.03GPa.Moreover,we observe that the lattice vibration peak occurs to split firstly,and then to overlap into one peak on increasing the pressure from 3.03 GPa to 7.13GPa.In Previous studies,Adams reported the discovery of CCl₄â…£at room temperature and above 2GPa,but Ebisuzaki¹² did not observed theâ…¢-â…£transition below 3GPa.In our experimental,the appearance of the new mode at 225cm^-1 is the obvious evidence of phase transition of the CCl₄â…¢-CCl₄â…£at about 3GPa,and the new mode perhaps may come from librational lattice modes of the CCl₄â…£.It is worth noting that at the range of 1.33-6.29GPa,the Fermi resonance interaction occurs to change and three peaks appear,two strong peaks at 755 cm^-1 and 769 cm^-1,a weak peak at 803 cm^-1,respectively.Because the frequency of the v₁ + v₄ combination is higher that of the v₃(T₂) fundamental after the pressure-induced frequency shifting under this compression range,the weak peak at 803cm^-1 is now the combination frequency(v₁+v₄).And the two strong peaks at 755 cm^-1 and 769 cm^-1are the fundamental v₃(T₂) due to splitting of the fundamental v₃(T₂) mode,although the spectral profiles of the two strong peaks are similar to the Fermi resonance modes at ambient pressure.Therefore,the relatively position of the v₁+v₄ combination and the v₃(T₂) fundamental interchanges at high pressure corresponding that at ambient pressure.The splitting of the fundamental v₃(T₂) mode indicates that the symmetry of the CCl₄ molecular reduces and the bond constant changes due to the effect of high pressure.Some Raman literatures exist for CCl₄ at pressures below 3GPa,however,little has been reported for the Raman spectra of CCl₄ at high pressure of above 3GPa.So the Raman spectra and the phase transition for the solid CCl₄ at high pressure are the main interest of this study by micro-region high-resolution Raman scattering.It can be seen from the Raman spectra for CCl₄ under 7.13-12.3GPa that all peaks continuously shift toward higher wavenumbers with increasing pressure.At the same time,we have observed the splitting of the v₂(E) and v₄(F₂) modes.Under 7.85GPa, the v₂(E) splits into two peaks distributed at 258 and 268 cm^-1.The v₄(F₂) splits into three peaks distributed at 320,336 and 351cm^-1 under 9.18GPa.Moreover,the combination frequency(v₁+v₄) disappears and the v₃(T₂) fundamental splits into three peaks above 7.13GPa.However,at the whole process of compression we have not observed the splitting of the C-Cl symmetric stretching mode v₁(A₁).The liquid CCl₄ belongs to T_d point group and the v₁ is not degeneracy,the v₂ is twofold degeneracy,the v₃ and v₄ are threefold degeneracy.Under 7.13-12.3GPa,we detect a doublet for v₂,triplets for v₃ and v₄,in agreement with group theory analysis.It shows that there is a solid -to- solid transition at this pressure range and the symmetry reduces for the new high-pressure phase.And the disappearance of the combination frequency(v₁+v₄) arises from the splitting of the v₃(T₂) and v₄(F₂) fundamental, and the change of Fermi resonance interaction.It suggest that the combination frequency(v₁+v₄) is due to second-order processes for the solid CCl₄ under high pressure.Nevertheless,the interpretation of the spectrum of the solid CCl₄ under high pressure is complicated due to fine structure arising from the crystal field effects as well as site splitting.The v₂,v₄ and v₁ show entirely positive pressure dependence at the whole pressure region.However,the slopes(dω/dP) of the v₂,v₄ and v₁ are various at different pressure region,and there are two sudden changes in the slope of frequency-pressure curve at 0.73GPa and 7.13GPa,respectively.On the other hand, the new modes appear at 3.03 GPa and about 7.13GPa.The observed changes in the Raman spectra,including sudden changes in the slope of frequency-pressure curve and appearance of new modes,indicate the phase transitions occur during the compress processes.Reference to the previous literatures,we identify that the solid-solid phase transition from the CCl₄â…¡to the CCl₄â…¢occurs at 0.73GPa,and the CCl₄â…¢undergoes a transformation below 3.03 GPa to the CCl₄â…£.Further the CCl₄â…£transforms in a new high-pressure phase at about 7.13GPa,and the symmetry of the new high-pressure phase is less than that of the CCl₄â…£.It should be pointed out that CCl₄â…£-new phase transition is gradually process at the pressure region from 6GPa to 9GPa,with the corresponding the splitting of modes v₂(E),v₄(F₂) and v₃(T₂).2.Effect of pressure on the Fermi resonance of benzeneThe high-pressure effects of benzene molecules Fermi resonance has been investigated by Raman scattering spectroscopy up to 13GPa.The experimental results show that the blue-shift and splitting of the Raman bands of benzene molecules occurs with the pressure increases,and the relative intensity of some bands also takes place significant changes.With the pressure increasing,the unperturbed peak separationΔ₀ of the v₁(ring breathing a_1g) + v₆(ring deformation e_2g) combination and the v₈ (ring stretching e_2g) fundamental for the benzene molecules progressively increases, and the intensity ratio of R_f/a of the two Fermi resonance peaks decreases with the coupling coefficientωthe Fermi resonance reducing.The Fermi resonance phenomenon disappears above the pressure of 11GPa.The increase of the Fermi level intervalsΔ₀ of the v₁(ring breathing a_1g) + v₆(ring deformation e_2g) combination and the v₈(ring stretching e_2g) fundamental results in the inherence frequency increasing,and the main reason of the Fermi resonance phenomenon gradually disappearing is the coupling coefficientωof the Fermi resonance reduces with the pressure increasing.3.Effect of pressure on the Fermi resonance of the binary solutions of CCl₄ and C₆H₆Raman spectra of the binary solutions of CCl₄ and C₆H₆ have been obtained at the range of 0-13GPa,and the frequency shift speed of CCl₄ and C₆H₆ liquid have been compared with the binary solutions,respectively.The results show that the frequency shift speed of CCl₄ and C₆H₆ in the binary solutions is higher than that of CCl₄ and C₆H₆ liquid,respectively.The molecule vibration characteristics in the binary solutions is different from that in the pure liquid,and the Raman frequency shift(blue shift) speed in the binary solutions increases comparing with the pure liquid.The Raman frequency shift of CCl₄,which has small density and large compression coefficient,is large than that of C₆H₆ which has large density and small compression coefficient.It indicates that The Raman frequency shift speed of molecular with variational compression coefficient is different.The pressure corresponding to the Raman band splitting decreases in the binary solutions than the pure liquid,and the Fermi resonance phenomenon disappears ahead of time since the influence of phase transition and the increase of frequency-shift speed in the binary solutions.4.Decompression ProcessIn the gradually process of decompression,the Raman spectra of CCl₄,C₆H₆ and the binary solutions of CCl₄ and C₆H₆ resembled those seen at comparable pressures during compression.It indicates that the solid-solid phase transition and the Fermi resonance phenomenon for all the samples are completely reversible under high pressure at room temperature.In summary,the study results provide an experimental evidence so that we can learn the phase transition and the molecular interactions under high pressures and the different environment.It enriches the studies of environmental effect on the Fermi resonance,and provides positive reference value for the spectral certification and assignment.Take advantage of these experimental results,the Raman spectra of frequency-shift speed and the expected frequency can be expected to obtain by choosing different solvent under high pressure.It provides also a method and ideas in order to explore the high pressure solvent effects.