In the framework of all-electron density functional theory, we present a comparative study of the pure carbon and BC_3 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 BC_3 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 percent. 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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