C 60 orientational ordering in superconducting Na 2RbC 60

Abstract

Rietveld refinements of x-ray powder profiles show that the C 60 molecules in Na 2RbC 60 are orientationally ordered in a manner essentially the same as in pure C 60, 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 Pa 3 (simple cubic) than in Fm 3 m (merohedral disorder) or by assuming uniform spherical shells of charge (complete disorder). The anomalously low T c of this compound cannot be attributed to excess orientational disorder, and therefore remains a mystery.