D. A. Neumann, Q. Huang, J. R. D. Copley, J. E. Fischer, P. A. Heiney, R. M. Strongin, L. Brard, and A. B. Smith, III, in Recent Advances in the Chemistry and Physics of Fullerenes and Related Materials, Vol. 2, edited by R. S. Ruoff and K. M. Kadish, p. 791. (The Electrochemical Society, Pennington NJ, 1995).
We report neutron powder diffraction measurements of orientational ordering for the 6,5-annulene (CS symmetry) isomer of C61D2. We confirm previous x-ray measurements which have shown that above 290 K this isomer has an fcc structure, with the molecules orientationally disordered such that the methylene groups preferentially occupy "octahedral" sites. Below 290 K, the diffraction pattern can be indexed on a simple cubic lattice, in direct analogy with the orientational ordering transition of C60. Refinements of data obtained below 290 K indicate that there are three different molecular orientations in this phase and that the fraction of the molecules which adopt the majority orientation is independent of temperature.