Pressure-Induced Phase Transitions of C70 Nanotubes

Single crystalline C-70 nanotubes having a face-centered-cubic (fcc) structure with diameters on a milometer scale were synthesized by a facile solution method. In situ high pressure Raman spectroscopy and X-ray diffraction have been employed to study the structural stability and phase transitions of the pristine sample. We show that the molecular orientation-related phase transition from the fcc structure to a rhombohedral structure occurs at about 1.5 GPa, which is similar to 1 GPa higher than in bulk C-70. Also, the C-70 molecules themselves are more stable in the nanotubes than in bulk crystals, manifested by a partial amorphization at similar to 20 GPa. The crystal structure of C-70 nanotubes could partially return to the initial structure after a pressure cycle above 30.8 GPa, and the C-70 molecules were intact up to 43 GPa. The bulk modulus of C-70 nanotubes is measured to be similar to 50 GPa, which is twice larger than that of bulk C-70.