C60 orientational ordering in superconducting Na2RbC60

Rietveld refinements of x-ray powder profiles show that the C60 molecules in Na2RbC60 are orientationally ordered in a manner essentially the same as in pure C60, the only fullerene-based superconductor thus far to do so. Fits to data at both 300 K and 27 K are significantly better in space group Pa3 (simple cubic) than in Fm3m (merohedral disorder) or by assuming uniform spherical shells of charge (complete disorder). The anomalously low Tc of this compound cannot be attributed to excess orientational disorder, and therefore remains a mystery.