Fulleride superconductors and orientational order: T vs lattice constant in Na2Rb Cs1−C60

The effect of orientational order on fulleride superconductivity was studied by performing DC SQUID magnetometry and high-resolution x-ray diffraction on the title quaternary phases. These are all simple cubic, Pa3space group, with the same molecular orientational order as occurs in pure C60. The Tc vs. lattice constant correlation is much steeper than is found for the merohedrally-disordered Fm3m phases, implying the possibility of large Tc enhancements with only minor additional dilation of the sc lattice. The effect of orientational order can be rationalized as primarily due to a density of states effect.