Elastic properties and pressure‐induced phase transitions of single‐walled carbon nanotubes

We studied the structure of single‐walled carbon nanotubes under hydrostatic pressure by first‐principles calculations. The circular tubes collapse at high pressure (7 GPa) to a phase with an elliptical cross‐section. The elliptical structure leads to the formation of diamond‐like bonds between the tubes at the point of strongest curvature. The bulk modulus of the circular, ambient‐pressure phase that we found from our calculations is very similar to graphite (37 GPa). The interlinked phase also shows graphite‐like behavior in the low‐ and high‐pressure range: between 3.5 and 6.5 GPa its compressibility is very large. We compare our theoretical predicitions to X‐ray scattering and piston–cylinder experiments.