Crystal Structure, Bonding, and Phase Transition of the Superconducting Na 2 CsC 60 Fulleride

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Abstract
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 [See equation in the PDF file], isostructural with pristine C60. Anticlockwise rotation of the C60 units by 98° about [111] 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 C60 molecule lengthens the hexagon-hexagon fusions and shortens the pentagon-hexagon fusions (to ∼1.43 angstroms). On heating, Na2CsC60 undergoes a phase transition to a face-centered-cubic [See equation in the PDF file] 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.