PRESSURE-INDUCED BAND-GAP REDUCTION, ORIENTATIONAL ORDERING AND REVERSIBLE AMORPHIZATION IN SINGLE-CRYSTALS OF C70 - PHOTOLUMINESCENCE AND RAMAN STUDIES

Photoluminescence and Raman scattering experiments have been carried o ut on single crystals of C70 up to 31 GPa to investigate the effect of pressure on the optical band gap, vibrational modes and stability of the molecule. The photoluminescence band shifts to lower energies and the pressure dependence of the band maxima yields the hydrostatic defo rmation potential to be 2.15 eV. The slope changes in the pressure dep endence of peak positions and linewidths of the Raman modes associated with the intramolecular vibrations at 1 GPa mark the known face-centr ed cubic-->rhombohedral orientational ordering transition. The reversi ble amorphization in C70 at P > 20 GPa has been compared with the irre versible amorphization in C60 at P > 22 GPa in terms of carbon-carbon distance between the neighbouring molecules at the threshold transitio n pressures, in conjunction with the interplay between the intermolecu lar and intramolecular interactions.