Comparative high pressure Raman studies on perfluorohexane and perfluoroheptane

• High pressure Raman spectroscopic studies on perfluorohexane and perfluoroheptane have been demonstrated.
• Pressure induced phase transitions have been indicated using ω, dω/dP order parameters.
• Solid I transition is observed in perfluoroheptane.
• Pressure induced behavior of mid-chain perfluorocarbon is different form mid-chain hydrocarbon.
• High pressure Raman studies on mid-chain perfluorocarbon till 12 GPa have been done for the first time.

High pressure Raman spectroscopic studies on perfluorohexane and perfluoroheptane have performed up to 12 GPa. Perfluorohexane under goes two pressure induced transitions: (1) liquid–solid transition at 1.6 GPa and (2) solid–solid transition at 8.2 GPa. On the contrary, perfluoroheptane under goes three phase transitions, they are as follows: (1) liquid–solid transition at 1.3 GPa, (2) intermediate solid I transition at 3 GPa, (3) solid II transition at 7 GPa. The change in slope (dω/dP) shows that the solid I transition at 3.0 GPa could be the conversion of mid-gauche defect into trans conformers for perfluoroheptane. The pressure induced Raman spectra and the behavior of individual band with pressure shows that the solid phase comprises more than one conformer beyond crystallization. The intensity ratio for both the compounds shows that the high pressure phase beyond 8.2 and 7.0 GPa tends to have close packing with distorted all-trans conformers.

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