title: Crystal Structure, Bonding, and Phase Transition of the Superconducting Na2CsC60 Fulleride.
abstract: The crystal structure of superconducting Na(2)CsC(60) 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 C(60). Anticlockwise rotation of the C(60) units by 98 degrees about [111] allows simultaneous optimization of C(60)-C(60) and alkali-fulleride interactions. Optimal Na(+)-C(60)(3-) 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 approximately 1.43 angstroms). On heating, Na(2)CsC(60) undergoes a phase transition to a face-centered-cubic [See equation in the PDF file] phase, best modeled as containing quasi-spherical C(60)(3-) 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.
authors.forename: K
authors.initials: K
authors.lastname: Prassides
authors.forename: C
authors.initials: C
authors.lastname: Christides
authors.forename: I M
authors.initials: IM
authors.lastname: Thomas
authors.forename: J
authors.initials: J
authors.lastname: Mizuki
authors.forename: K
authors.initials: K
authors.lastname: Tanigaki
authors.forename: I
authors.initials: I
authors.lastname: Hirosawa
authors.forename: T W
authors.initials: TW
authors.lastname: Ebbesen