Phase diagram of single-wall carbon nanotube crystals under hydrostatic pressure

In a number of studies it has been shown that single-wall carbon nanotube crystals undergo phase transformations and shape changes as a function of nanotube radius and hydrostatic compression. When large radius nanotubes form a crystal, the circular tube cross section deforms to a hexagonal shape, while under hydrostatic pressure the cross section can transform from circular to oval or to a so-called racetrack shape. At still higher pressures cross linking of nanotubes has been inferred. A model which explains and describes these phenomena is presented and a phase diagram is computed. It is shown that the intertube interaction plays a dominant role at low pressure only, and that the curvature energy is important over the whole pressure range. The model and the general conclusions should apply also to other nanotube materials such as those based on BN and MoS₂.