Polygonization of single-wall carbon nanotube bundles under high pressure

The pressure-induced structural changes of single-wall carbon nanotubes organized into two-dimensional crystalline bundles are studied by neutron diffraction up to $\sim 50\hspace{0.5em}\mathrm{kbar}.$ The strong variation of the (10) Bragg peak position with pressure is interpreted as a progressive deformation of the tube section from circular to hexagonal, in addition to the van der Waals compression. Data at 50 kbar suggest that the tubes are closed to be fully polygonized at this pressure. The anisotropic behavior of the patterns with pressure is assigned to the presence of a uniaxial pressure component and to preferential orientations of the bundles.