In the framework of all-electron density functional theory, we present a comparative study of the pure carbon and BC3 compound nanotubes containing different kinds of topological defects (seven-, eight- and nine-membered rings) under uniaxial tensions. The formation energies of the topological defects for pure carbon nanotubes are significantly higher than those for BC3 compound nanotubes. For both pure and compound nanotubes, sidewall defects by seven- and eight-membered rings become energetically preferred to form when the uniaxial strain approaches about 6.5%. In contrast, the total energy of the nanotube with a nine-membered ring defect is always much higher than the others. The formation mechanism of a Stone-Wales (5-7-7-5) defect in the pure carbon nanotubes is studied and we find that the barrier energy for the formation of a defect decreases monotonically with increasing strain.
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