STRUCTURE AND PHASE-TRANSITION OF THE 6,5-ANNULENE ISOMER OF C61H2

We have used differential scanning calorimetry and x-ray diffraction t o 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 angs trom. Below T(c) = 290 K the symmetry is lowered to Pa3BAR, a simple c ubic structure. At 20 K the lattice parameter is 14.06 +/- 0.02 angstr om, with substantial statistical disorder remaining. The transition te mperature in C61H2 is slightly higher than in C60O and higher still th an in C60, but the three compounds exhibit orientationally ordering tr ansitions between qualitatively similar phases.