Pressure-induced band gap reduction, orientational ordering and reversible amorphization in single crystals of C₇₀: photoluminescence and Raman studies

Abstract

Photoluminescence and Raman scattering experiments have been carried out 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 deformation potential to be 2.15 eV. The slope changes in the pressure dependence of peak positions and linewidths of the Raman modes associated with the intramolecular vibrations at 1 GPa mark the known face-centred cubic→orhombohedral orientational ordering transition. The reversible amorphization in C₇₀ at P > 20 GPa has been compared with the irreversible amorphization in C₆₀ at P > 22 GPa in terms of carbon-carbon distance between the neighbouring molecules at the threshold transition pressures, in conjunction with the interplay between the intermolecular and intramolecular interactions.