Crystal Structure, Bonding, and Phase Transition of the Superconducting Na2CsC60 Fulleride

Science  18 Feb 1994:
Vol. 263, Issue 5149, pp. 950-954
DOI: 10.1126/science.263.5149.950


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.