A. N. Lommen, P. A. Heiney, B. G. M. Vaughan, P. W. Stephens, D. F. Liu, D. Li, A. L. Smith, A. R. McGhie, R. M. Strongin, L. Brard, and A. B. Smith, III, Phys. Rev. B49, 12572-12577 (1994).
We have used differential scanning calorimetry and x-ray diffraction to study the crystalline phases of the 6,5-annulene isomer of C61H2. At 308 K the methylenated fullerenes are orientationally disordered in a face-centered cubic lattice with lattice parameter 14.19 +/- 0.02 Angstrom. Below Tc = 290 K the symmetry is lowered to Pa3-bar, a simple-cubic structure. At 20 K the lattice parameter is 14.06 +/- 0.02 Angstrom, with substantial statistical disorder remaining. The transition temperature in C61H2 is slightly higher than in C60-O and higher still than in c60, but the three compounds exhibit orientationally ordering transitions between qualitatively similar phases.
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