Structure and phase transitions of single-wall carbon nanotube bundles under hydrostatic pressure

We have studied the structures of single-wall carbon nanotube bundles with and without hydrostatic pressure by using constant pressure molecular dynamics methods. The structure is found to be strongly dependent on the symmetry of the tubes, and only $(6n,6n)$ tubes could be assembled into an ideal hexagonal lattice. With increasing pressure, all tube lattices undergo a structure transition, while the transition pressure varies with the symmetry and radius of the tube.

Figure 1

Schematic illustration of the interface structure of the neighboring tubes in SWCNTB lattices. Solid lines represent one graphitelike plane and the dashed lines another graphitelike plane of the neighboring tube. Along the armchair direction, the lattice slides $1∕3$ period ($AB$ in one plane and $A^{\prime }{B}^{\prime }$ in another), and along the zigzag direction, the lattice slides $1∕2$ period along the perpendicular direction ($CD$ in one plane and $C^{\prime }{D}^{\prime }$ in another).

Figure 2

Pressure vs volume for SWCNTB’s. The discontinuity of each curve suggests that a phase transition occurred. Due to its much larger transition pressure, the result of the (6,6) SWCNTB lattice is not shown. Lines are shown to guide the eyes.

Figure 3

Transition pressure $P_t$ vs the radius of tube. (6,6), (12,12), and (15,15) SWCNTB lattices (solid squares) have larger transition pressures than those of isolated single-wall carbon nanotubes, while (8,8), (10,10), (24,0), and (30,0) SWCNTB lattices (solid circles) have the lower ones. The blank circles are the transition pressures of (6,6), (8,8), (10,10), (12,12), and (20,20) isolated SWCNTB’s (Ref. 18), respectively. The transition pressure can be well fitted to $\sim 1∕R^3$ for the solid or blank circles.

Figure 4

Snapshots of the cross section of (10,10) SWCNTB’s under pressure of (a) $0.6\mathrm{GPa}$, (b) $0.7\mathrm{GPa}$, (c) $0.85\mathrm{GPa}$, and (d) $1.0\mathrm{GPa}$. A new metastable state of AB sequence under pressure $0.8–0.93\mathrm{GPa}$ with A and B representing the dumbbell-like and ellipticlike nanotubes, respectively, is observed.