Molecular dynamics simulations of phase transitions in argon-filled single-walled carbon nanotube bundles under high pressure

The behavior of single-walled carbon nanotubes has been investigated under high pressures with the help of classical molecular dynamics simulations in two configurations: when bundles are empty and when argon is present as a pressure transmitting medium. Our calculations show that for the empty tubes, depending on the pressure step, relaxation times, and temperature, several different organizations of collapsed tubes exist for the high-pressure phase above 2.4 GPa. When the nanotubes are filled with argon (as well as surrounded by it), the high-pressure behavior is found to be substantially different. The phase transition shifts to higher pressures as the number of argon atoms inside the nanotubes is increased beyond a critical value and becomes close to 7 GPa for the calculated optimum Ar density. Computed x-ray diffraction patterns of argon-filled nanotubes show that the intensity of the first diffraction peak, which experimentally has been taken as indicative of two-dimensional order in bundles, persists up to higher pressures. We propose that seemingly varied experimental observations in the high-pressure phase transitions of carbon nanotubes are due to the pressure transmitting medium at different densities.