We have investigated the effect of pressure on vibrational modes of a c60/c70powder mixture by Raman spectroscopy. Pressure coefficients for 16 well-resolved peaks were obtained. We find negative slopes for two modes in the intermediate frequency range, and a sign change of the slope for the 1186 cm#x2212;1(C70) mode at 4 GPa. A nonreversible change to amorphous carbon is found to occur starting at about 10 GPa, with complete transition to amorphous carbon at about 22 GPa, and no further transition up to 36 GPa.
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