Superconductivity in the Fullerenes

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

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.

Mechanism of superconductivity in K3C60

Using electronic states and phonon states from the first-principles calculations and including both conventional electron-phonon charge coupling and Jahn-Teller coupling, we predict Tc and other superconducting properties. The only adjustable parameter in the theory is the screening length, Rsc. Using Rsc = 0.8-1.0 A, we find excellent agreement with experiment for Tc (16-18 K), pressure dependence of Tc (Delta Tc = -6 to -10 K for 1 GPa), and 12 C to 13 C isotope shift (alphaC = 0.2); experimental values: 19 K, -7 K, and 0.3, respectively.