Raman scattering studies on n-heptane under high pressure.

Area of Science:

• Materials Science
• Condensed Matter Physics
• Spectroscopy

Background:

• n-heptane exhibits complex behavior under high pressure.
• Previous studies have identified a liquid-solid transition but lacked detailed insights into higher-pressure solid phases.

Purpose of the Study:

• To investigate the high-pressure phase behavior of n-heptane using Raman spectroscopy.
• To identify and characterize pressure-induced phase transitions beyond the liquid-solid transition.
• To elucidate the molecular mechanisms driving these transitions.

Main Methods:

• High-pressure Raman scattering experiments were conducted at ambient temperature.
• Raman spectra were analyzed to detect changes in vibrational modes and phase transitions.
• Pressure-induced structural and conformational changes were inferred from spectral data.

Main Results:

• A liquid-solid transition was confirmed around 1.5 GPa.
• A novel solid-solid phase transition was observed at 7.5 GPa, marked by distinct spectral changes.
• Dampening of methyl group rotation was observed below 7.5 GPa, consistent with theoretical predictions.
• Above 7.5 GPa, a conversion from gauche to trans conformations in the solid phase was definitively observed.
• The absence of hysteresis suggests a second-order or no-volume-change transition.

Conclusions:

• The observed 7.5 GPa transition is proposed to be an orientational order-disorder transition.
• This transition is driven by the pressure-induced dampening of methyl group rotation.
• The findings provide new insights into the high-pressure molecular behavior of n-alkanes.