ORIENTATIONAL PHASE-TRANSITIONS IN A C-70 CRYSTAL AT HIGH-PRESSURE

The Raman spectra of C70 crystals have been studied at pressures up to 17 GPa. The frequency OMEGA of most of the optical vibrations increas es linearly with the pressure P. The pressure-induced shift partial-de rivative OMEGA/partial-derivative P itself has jumps at P1 congruent-t o 2+/-0.2 and P2 congruent-to 5.5+/-0.5 GPa. The jumps observed in par tial-derivative OMEGA/partial-derivative P are correlated with a sharp change in the half-width of the Raman line for several optical modes. These spectral features of the Raman scattering are linked with phase transitions caused by an orientational ordering of the C70 molecules in the crystal lattice as the pressure is raised.