High Pressure Raman Study of Hexafluorobenzene Crystals

Abstract

Pressure effects on the Raman active inter- and intramolecular vibrations of hexafluorobenzene crystals were studied under hydrostatic pressures up to about 5 GPa at room temperature in a gasketed diamond anvil cell. The observed Raman spectra indicate that very gradual application of pressure to hexafluorobenzene liquid up to about 0.3 GPa gives rise to supercompressed liquid and the supercompressed liquid crystallizes in phase I by further successive applications of pressure. On the other hand, hexafluorobenzene liquid crystallizes in phase II by quick application of pressure up to about 0.8 GPa. The crystal structure in phase I grown via supercompression is unchanged up to about 2—4 GPa, depending on the experimental conditions and finally transfers to phase II. Coexistence of the crystal phases I and II gives complex spectral structures in both the inter- and intramolecular vibrational regions under various pressures. The pressure-induced frequency shift of the intermolecular vibrations of hexafluorobenzene crystal is compared with those of hexachloro- and hexabromobenzene crystals and it is shown that the repulsive force between the bromine atoms belonging to different molecules is stronger than the repulsive forces between the fluorine atoms and also between the chlorine atoms.

Hexafluorobenzene under Extreme Conditions

“…As pressure is increased, all modes shift toward higher energies as expected. Above ambient, the e 1g fundamental mode splits (and shifts) at 0.4 GPa, which corroborates the transition from liquid into phase I observed in ref . At or above 0.4 GPa, two peaks emerge near 100 and 120 cm –1 respectively heralding the transition into phase II .…”

“…Above ambient, the e 1g fundamental mode splits (and shifts) at 0.4 GPa, which corroborates the transition from liquid into phase I observed in ref . At or above 0.4 GPa, two peaks emerge near 100 and 120 cm –1 respectively heralding the transition into phase II . Above 5.7 GPa, a new peak emerges near 420 cm –1 .…”

“…We have performed three studies (Raman, XRD, and IR) of an important and fundamental solvent, C 6 F 6 , to continue our studies of fundamental hydrocarbon and fluorocarbon ringed systems including, to the best of our knowledge, the first reported IR and XRD studies of hexafluorobenzene at high pressure. We have confirmed the presence of phase I near 0.3 GPa and phase II near 2 GPa which were observed in an earlier Raman high pressure study . We have observed evidence for three new phase transitions of C 6 F 6 : phase III just above 8.8 GPa, phase IV above 12 GPa, and phase V above 20.8 GPa.…”

“…At ambient temperature, C 6 F 6 undergoes known phase transitions from the liquid state with pressure (liquid → phase I → phase II) up to 4.7 GPa . The liquid solidifies into phase I when slowly compressed beyond 0.3 GPa and then transforms into phase II somewhere between 2 and 4 GPa . If, on the other hand, the material is rapidly compressed up to 0.8 GPa, the material goes directly into phase II .…”

Understanding congruent melting in binary solids: Molecular models of benzene–hexafluorobenzene mixtures

“…The experimental solid structure determined by x-ray diffraction was measured at 120 K while the normal melting point is 278 K. Researchers using Raman spectroscopy have found more than one hexafluorobenzene solid phase as a function of pressure. 35 Other researchers propose that the effective shape of the fluorine atoms may not be spherical but elliptical. 36 The type of solid-fluid phase diagram shown here is characteristic of other aromatic mixtures.…”