No abstract

No abstract

Heiney et aL Reply:We agree that the Pa 3 space group gives a better representation of the orientationally ordered phase of solid Ceo than the Pn3 group which we used in our original analysis [1]. We are pleased that the improved fit to the low-temperature data confirms the central point of our paper, which was that solid C^o undergoes a structural phase transition at 249 K from an orientationally disordered plastic crystal to an orientationally ordered crystal with some residual orientational disorder (static or dynamic).The improved fit described in the Comment [2] is still not perfect. The intensities of seven reflections in the range 29° < 20<49^ are still significantly underestimated and two are overestimated by the model. We believe that the remaining discrepancies between the Pa 3 model and our original data [1] are due to stacking-fault defects in the original sample [3]. If the same Pa 3 model (with residual disorder) is applied to data from a well-annealed sample with lower stacking-fault density [4], an even more convincing fit is obtained.

The high-temperature structure of solvent-free C(70) has been determined with high-resolution x-ray powder difraction and electron microscopy. Samples crystallized from solution form hexagonal close-packed crystals that retain an appreciable amount of residual toluene, even after prolonged heating. Samples prepared by sublimation, which contain no detectable solvent, are primarily face-centered cubic with some admixture of a hexagonal phase. The relative volume of the hexagonal phase can be further reduced by annealing. The structures of both phases are described by a model of complete orientational disorder. The cubic phase contains an appreciable density of stacking faults along the [111] direction.

No abstract