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-princi-ples 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 experi-ments. 1. Introduction The high-pressure behavior of carbon nanotubes has been studied with a variety of experimental techniques. Most of these studies seemed to indicate a phase transition of carbon under pressure, e.g., by X-ray [1, 2] a change in the bulk compressibility [3], the loss of the Raman [4–7] and absorption [8] intensity, or an increase in the resistivity [9, 10] under pressure. Although the experimental studies agree in some form of change of the ambient pressure phase, there is remarkabl