STRUCTURE, DYNAMICS, AND PHASE-TRANSITIONS IN THE FULLERENE DERIVATIVES C60O AND C61H2

The effect of perturbing the icosohedral symmetry of C-60 by the addit ion of the side groups -O and -CH2 upon orientational order-disorder a nd glass transitions in solid C-60 has been studied by a combination o f high-resolution capacitance dilatometry and single-crystal x-ray and powder inelastic neutron scattering. Both fullerene derivatives C60O (epoxide) and C61H2 (6,5-annulene) are shown to undergo a sequence of transitions similar to that found in pure C-60, i.e., a first-order or ientational ordering transition just below room temperature followed b y an orientational glass transition at lower temperatures. Although th e exact origin of the glass transition in C61H2 is unclear, the glass transition in C60O has the same origin as that in C-60, with a signifi cantly higher degree of order due to a larger energy difference betwee n pentagon and hexagon orientations. Tire dilatometric data at the gla ss transition indicate that, in contrast to C-60, the ground-state ori entation of both C60O and C61H2 molecules is that with the smallest vo lume, also demonstrating a significant influence of the side groups up on the derails of the structure, A possible explanation oft these diff erences in terms of steric effects is proposed.