Orientational phase transition inNaxC60(1<x<3)

X-ray diffraction and calorimetry data on cubic ${\mathrm{Na}}_{\mathit{x}}$ ${\mathrm{C}}_{60}$(1<x<3) reveal orientational ordering transitions at ${\mathit{T}}_{\mathit{m}}$ above that of pure ${\mathrm{C}}_{60}$, e.g., ${\mathit{T}}_{\mathit{m}}$(x=1.3)=325 K. The ordered phases are the same as in pure ${\mathrm{C}}_{60}$: 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 ${\mathrm{C}}_{60}$ molecules and and Na ions which we calculate from the local charge density of ${\mathrm{C}}_{60}$.