A New 3D Carbon Allotrope Composed of Penta-graphene Nanotubes with Low Lattice Thermal Conductivity.

Motivated by the unique geometries and novel properties of penta-graphene (PG) and its derivatives, we propose a new stable 3D carbon allotrope, penta-C72, which is composed of PG nanotubes by connecting adjacent tubes in a tip-to-tip manner. Using first-principles calculations, we confirm its dynamical, thermal, and mechanical stabilities and find that penta-C72 is semiconducting with an indirect bandgap of 2.12 eV. Its lattice thermal conductivities at 300 K are found to be anisotropic with values of 97.32 and 179.35 W/mK along the x and z directions, respectively, which are much lower than that of diamond (2664.93 W/mK) and carbon nanotube-based bct-C4 (1411.02 W/mK). A detailed analysis of both harmonic and anharmonic properties suggests that the soften acoustic phonon modes, the low Young's modulus, and strong anharmonicity are the key factors for the low lattice thermal conductivity. The study expands the family of carbon materials by assembling PG nanotubes.