Electron-phonon interaction in the solid form of the smallest fullerene C20

The electron-phonon coupling of a theoretically devised carbon phase made by assembling the smallest fullerenes C-20 is calculated from first principles. The structure consists of C-20 cages in a fcc lattice interlinked by two bridging carbon atoms in the interstitial tetrahedral sites (fcc-C-22). The crystal is insulating but can be made metallic by doping with interstitial alkali atoms. In the compound NaC22 the calculated coupling constant lambda/N(0) is 0.28 eV, a value much larger than in C-60, as expected from the larger curvature of C-20. On the basis of McMillan's formula, the calculated lambda = 1.12 and a mu* assumed in the range 0.3-0.1, a superconducting T-c in the range 15-55 K is predicted.