X-ray diffraction study of carbon nanotubes under high pressure

The structure and physical properties of carbon nanotubes have been investigated by using in situ high pressure energy dispersive x-ray diffraction with synchrotron radiation at pressures up to 50.7GPa. At atmospheric pressure, the structure of carbon nanotubes is similar to the hexagonal close-packed lattice of graphite with the interplanar spacing of the diffraction line (002) d(002) = 0.3404 nm and that of the line (100) d(100) = 0.2116 nm. According to the high pressure x-ray diffraction results, the diffraction line (002) is broadened and weakened above 8GPa, and carbon nanotubes become partly amorphous. When the pressures of 10 and 20GPa are decreased to zero, the diffraction line (002) is partly recovered. While at the maximum pressure of 50.7GPa, they become entirely amorphous and this amorphous transition is irreversible. We used the equation of state of Birch-Murnaghan to fit the P-V data of carbon nanotubes and obtained the bulk modulus K-0 = 54.3 +/- 3.2 GPa (at K-0' = 4.0).