Superconductivity at 8.4 K in calcium-doped C60

• DOI: 10.1038/355529a0
• Publication Date: 2003-06-12T23:27:05Z
• Authors: A. R. Kortan; N. Kopylov; S. Glarum; E. M. Gyorgy; A. P. Ramirez; R. M. Fleming; F. A. Thiel; R. C. Haddon
• Keywords: Tetragonal crystal system; Cubic crystal system; Interstitial defect; Divalent; Lattice (music)

IT has been demonstrated1–6 that solid C60 can be readily intercalated with group IA alkali metals to give metallic or insulating compounds, depending on the dopant concentration. The metal atoms diffuse into tetrahedral and octahedral interstitial sites of the C60 lattice with little disturbance to the face-centred cubic (f.c.c.) packing7. The A3C60f.c.c. phases (A is K, Rb, Cs and mixtures of these) exhibit superconductivity with a transition temperature that increases with lattice constant8. At higher dopant concentration a body-centred tetragonal A4C60 phase9 and an insulating, body-centred cubic A4C60 phase10 are found. Here we report that the divalent group IIA intercalant calcium can be intercalated into the f.c.c. sites of C60 to form a solid solution, and that, near a Ca:C60 ratio of 5:1, a phase transformation occurs to a simple cubic phase. Measurements of microwave loss, magnetic susceptibility and Meissner effect show that the simple cubic phase becomes superconducting below 8.4 K.