Low lattice thermal conductivity of a 5-8-peanut-shaped carbon nanotube

5-8-defects are well-known in graphene and other 2D carbon structures, but not well-studied in one dimensional (1D) carbon materials. Here, we design a peanut-shaped carbon nanotube by assembling the 5-8-cage composed of carbon 5- and 8-membered rings, named 5-8-PSNT. Using first-principles calculations and molecular dynamics simulations, we find that 5-8-PSNT is not only thermally and dynamically stable, but also metallic. Moreover, its lattice thermal conductivity is only 95.87 W m-1 K-1, which is less than one tenth of the value of (6, 6) carbon nanotube that has a radius similar to that of 5-8-PSNT. A further analysis of the phonon properties reveals that the low lattice thermal conductivity of 5-8-PSNT arises from its low phonon group velocity, short relaxation time, large lattice vibrational mismatch and strong anharmonicity. These findings further suggest that a pentagon and an octagon as structural units can effectively modulate the properties of carbon materials.