C_<70> Molecular Stumbling inside Single-Walled Carbon Nanotubes

We report the structural study of C_-one-dimensional (1D) crystal formed inside single-walled carbon nanotubes (SWNTs). X-ray diffraction measurements were performed between 100 K and 999 K on C_-encapsulated SWNTs with an average diameter of 1.37 nm. Two different domains, where the dominant alignment of C_ molecular long axis is standing or lying with respect to the tube axis, were observed; the ratio of the standing to the lying C_-domains is roughly 7:3. Thermal expansion of interfullerene distance gave no evidence for orientational phase transitions, not as in the 3D crystals. Instead, the long-range order of the standing C_-alignment was thermally destroyed with an activation energy of 39&#177;4 meV. The results imply the importance of either the 1D fluctuation or the tube-C_ interaction at finite temperatures.