X-ray diffraction and calorimetry data on cubic NaxC60 (1 < x < 3) rev
eal orientational ordering transitions at T(m) above that of pure C60,
e.g., T(m)(x = 1.3) = 325 K. The ordered phases are the same as in pu
re C60: simple cubic, space group Pa3BAR, 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 Co
ulomb interactions between C60 molecules and Na ions which we calculat
e from the local density approximation charge density of C60.