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 pi 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 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.