C-60 ORIENTATIONAL ORDERING IN SUPERCONDUCTING NA2RBC60

Rietveld refinements of x-ray powder profiles show that the C60 molecu les in Na2RbC60 are orientationally ordered in a manner essentially th e same as in pure C60, the only fullerene-based superconductor thus fa r to do so. Fits to data at both 300 K and 27 K are significantly bett er in space group Pa3BAR (simple cubic) than in Fm3mBAR (merohedral di sorder) or by assuming uniform spherical shells of charge (complete di sorder). The anomalously low T(c) of this compound cannot be attribute d to excess orientational disorder, and therefore remains a mystery.