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A sample of C60 containing ca. 9% Kr@C60 has been used to form crystalline (0.09Kr@C60/0.91C60).(NiII(OEP)).2C6H6 whose X-ray crystal structure reveals that the Kr atom is centered within the carbon cage and does not produce a detectable change in the size of the fullerene.

A substantial amount of endohedral fullerene, Ar@C 60 (41 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, 13 C 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 (DT c ) in both K 3 (Ar@C 60 ) and Rb 3 (Ar@C 60 ) compared with pristine compounds (DT c 41 K and 42 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.

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