ORIENTATIONAL PHASE-TRANSITION IN NAXC60 (1-LESS-THAN-X-LESS-THAN-3)

X-ray diffraction and calorimetry data on cubic NaxC60 (1 < x < 3) reveal 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 pure 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 Coulomb interactions between C60 molecules and Na ions which we calculate from the local density approximation charge density of C60.