The crystal structure of superconducting Na2CsC60 was studied by high-resolution powder neutron diffraction between 1.6 and 425 K. Contrary to the literature, the structure at low temperatures is primitive cubic (Pa3), isostructural with pristine C60. Anticlockwise rotation of the C60 units by 98° about  allows simultaneous optimization of C60-C60 and alkali-fulleride interactions. Optimal Na+-C603- coordination is achieved with each sodium ion located above one hexagon face and three hexagon-hexagon fusions of neighboring fulleride ions (coordination number 12). Reduction of the C 60 molecule lengthens the hexagon-hexagon fusions and shortens the pentagon-hexagon fusions (to ∼1.43 angstroms). On heating, Na 2CsC60 undergoes a phase transition to a face-centered-cubic Fm3m phase, best modeled as containing quasi-spherical C603- ions. The modified structure and intermolecular potential provide an additional dimension to the behavior of superconducting fullerides and should sensitively affect their electronic and conducting properties.
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