Superconductivity of doped Ar@C₆₀

A substantial amount of endohedral fullerene, Ar@C₆₀ (>1 mg), was successfully obtained by heat treatment of C₆₀ powder under high pressure Ar atmosphere, followed by sequential HPLC separation. The isolation of pure Ar@C₆₀ was demonstrated by TOF-MS, ¹³C NMR spectroscopy and powder X-ray diffraction. We report here the synthesis of the first endohedral fullerene superconductors, K₃(Ar@C₆₀) and Rb₃(Ar@C₆₀) having critical temperatures of about 18 and 27 K, respectively. The decrease of T_c (ΔT_c) in both K₃(Ar@C₆₀) and Rb₃(Ar@C₆₀) compared with pristine compounds (ΔT_c >1 K and >2 K, respectively) is rather large, implying that the influence of encapsulated Ar atom on the C₆₀ 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@C₆₀ and C₆₀ have different unit cell parameters therefore different van der Waals radii.