Structure, dynamics and phase transitions in the fullerene derivatives C60O and C61H2

The effect of perturbing the icosohedral symmetry of ${\mathrm{C}}_{60}$ by the addition of the side groups -O and -${\mathrm{CH}}_{2}$ upon orientational order-disorder and glass transitions in solid ${\mathrm{C}}_{60}$ has been studied by a combination of high-resolution capacitance dilatometry and single-crystal x-ray and powder inelastic neutron scattering. Both fullerene derivatives ${\mathrm{C}}_{60}$O (epoxide) and ${\mathrm{C}}_{61}$${\mathrm{H}}_{2}$ (6,5-annulene) are shown to undergo a sequence of transitions similar to that found in pure ${\mathrm{C}}_{60}$, i.e., a first-order orientational ordering transition just below room temperature followed by an orientational glass transition at lower temperatures. Although the exact origin of the glass transition in ${\mathrm{C}}_{61}$${\mathrm{H}}_{2}$ is unclear, the glass transition in ${\mathrm{C}}_{60}$O has the same origin as that in ${\mathrm{C}}_{60}$, with a significantly higher degree of order due to a larger energy difference between pentagon and hexagon orientations. The dilatometric data at the glass transition indicate that, in contrast to ${\mathrm{C}}_{60}$, the ground-state orientation of both ${\mathrm{C}}_{60}$O and ${\mathrm{C}}_{61}$${\mathrm{H}}_{2}$ molecules is that with the smallest volume, also demonstrating a significant influence of the side groups upon the details of the structure. A possible explanation of these differences in terms of steric effects is proposed. \textcopyright{} 1996 The American Physical Society.