Superconductivity of doped Ar@C60

A substantial amount of endohedral fullerene, Ar@C-60 (> 1 mg), was successfully obtained by heat treatment of C-60 powder under high pressure Ar atmosphere, followed by sequential HPLC separation. The isolation of pure Ar@C-60 was demonstrated by TOF-MS, C-13 NMR spectroscopy and powder X-ray diffraction. We report here the synthesis of the first endohedral fullerene superconductors, K-3(Ar@C-60) and Rb-3(Ar@C-60) having critical temperatures of about 18 and 27 K, respectively. The decrease of T-c (Delta T-c) in both K-3(Ar@C-60) and Rb-3(Ar@C-60) compared with pristine compounds (Delta T-c > 1 K and > 2 K, respectively) is rather large, implying that the influence of encapsulated Ar atom on the C-60 cage and therefore to the superconductivity of its derivatives, is more pronounced than expected. Indeed, synchrotron X-ray powder diffraction results showed, rather unexpectedly, that Ar@C-60 and C-60 have different unit cell parameters therefore different van der Waals radii.