Thermal expansion coefficient in single-walled carbon nanotube bundles was determined as $(-0.15\pm 0.20)\times {10}^{-5}\hspace{0.5em}(1/\mathrm{K})$ for the tube diameter and $(0.75\mathrm{}\pm 0.25)\times {10}^{-5}\hspace{0.5em}(1/\mathrm{K})$ for the triangular lattice constant by means of x-ray scattering between 300 K to 950 K. The value for the intertube gap was $(4.2\mathrm{}\pm 1.4)\times {10}^{-5}\hspace{0.5em}(1/\mathrm{K}),$ which is larger than $2.6\times {10}^{-5}\hspace{0.5em}(1/\mathrm{K})$ for the c-axis thermal expansion in graphite. The results reveal the presence of a remarkably larger lattice anharmonicity in nanotube bundles than that of graphite. The small value for the tube diameter is consistent with the seamless tube structure formed by a strong covalent bond between carbon atoms comparable to that in graphite.

• Received 28 May 2001

DOI:https://doi.org/10.1103/PhysRevB.64.241402