ORIENTATIONAL ORDERING IN THE SOLID FULLERENE OXIDE - C60O

Constant-pressure molecular dynamics simulations have been employed to investigate the orientational ordering in the solid phases of fullerene oxide, C60O. A pairwise additive atom-atom intermolecular potential model developed for solid C-60 is modified slightly to reflect the functionalized character of the C60O molecule. The simulation results indicate that at low temperature the carbon cages are frozen into a Pa(3) over bar-like structure, as in the pure C-60 solid. Most oxygen atoms point randomly to one of the neighbouring octahedral interstitial sites (i.e: [100] direction) but about 20% point to the smaller tetrahedral sites ([111] direction). Above the transition temperature, estimated to be around 210 K compared with the measured value of 278 +/- 2 K, C60O molecules rotate about the centre-of-mass-oxygen axis. The bridging oxygen atoms tend to wobble in their interstitial sites rather freely but they cannot move from one pocket to another. However, at very high temperature (ca. 800 K), the oxygen atoms are able to hop between different interstitial sites on the molecular dynamics timescale.