Raman study of pressure-induced structural change in C6I6 up to 47 GPa

The pressure change in the frequencies of intramolecular vibrational modes in hexaiodobenzene (C6I6) has been investigated by Raman scattering measurements at room temperature under pressure up to 47 GPa with a diamond anvil cell. An out-of-plane vertical vibration mode of carbon born structure produces mode softening by loading up to 13 GPa in spite of mode-hardening occurring in the other three modes related to the in-plane vibrations. This result is consistent with the molecular deformation observed by previous high pressure x-ray diffraction. The structure of the spectrum is obliterated with a further rise in pressure beyond 13 GPa due to an increase in the absorption of an excitation laser light. Raman peaks reappear at pressures below 4 GPa by releasing the pressure from 47 GPa; however, these peaks are located at different frequency regions from the original positions, indicating that molecular dissociation takes place during the pressure release.