Structural phase transition in carbon nanotube bundles under pressure

The Raman shifts as functions of pressure were measured for the radial breathing mode and tangential modes in carbon nanoropes containing (10,10) and (17,0) single-walled tubes. An abrupt decrease in the rate of change of the Raman shift with pressure for the tangential modes, and the disappearance of the radial breathing mode, were observed at approximately 1.7 GPa. These changes in the Raman spectrum result from a structural phase transition as the nanoropes are compressed. Theoretical calculations using the empirical force method confirm this transition, as indicated by a calculated discontinuity in the lattice constants and unit cell volume at 1.7 GPa.