Orientational phase transition in NaxC60 (1<x<3).

X-ray diffraction and calorimetry data on cubic NaxC60(1<xTm above that of pure C60, e.g., Tm(x=1.3)=325 K. The ordered phases are the same as in pure C60: simple cubic, space group Pa3 ̄, but the orientations in the disordered phase are more restricted. We explain how Na stabilizes the ordered phase to rather high T, while K and Rb do not, in terms of Coulomb interactions between C60 molecules and and Na ions which we calculate from the local density approximation charge density of C60.