Carbon nanotube bundles under high pressure: Transformation to low-symmetry structures

Structure transformations for crystalline bundles of single walled carbon nanotubes (10,10), (8,8), and (6,6), in response to external pressure are modeled by first-principles calculations. Upon pressure, the circular tube section is first transformed into an elliptical shape. Further pressure then leads to a flattened shape, similar to a 400-m track, with two flat sections connected by two cap sections. While the stress is taken up at the cap sections by bond buckling, the conjugate π bonding on the two flat sections becomes more effective and provides some stabilization for the structure. Such a transformation effectively squeezes the empty space inside a tube and thus reduces the intertube van der Waals repulsion. Collapse of the tube structures or linking between tubes via ${\mathrm{sp}}^3$ bonding is not observed up to a stress level of 20 GPa. Hexagonal tube sections are also observed, which is a metastable state, due to the the symmetry constraint of the triangular lattice during structure optimization. Such a structure is not favored as it is too rigid to adapt to external pressures.