THE synthesis of fullerenes encapsulating various metal atoms within the carbon cage (endohedral metallofullerenes) has stimulated wide interest 1,2 because of their unusual structural and electronic properties. Most of the metallofullerenes prepared so far have been based on C82, and have incorporated lanthanum1,3-5, yttrium6,7, scandium8-10 and most of the lanthanide elements11,12'. Although there has been some debate about the endohedral nature of these compounds2,13,14, observations using scanning tunnelling microscopy15,16, extended X-ray absorption fine structure 17,18, transmission electron microscopy19 and electron spin resonance3,6-8,10 have strongly suggested that the metal atoms are indeed inside the fullerene cages; theoretical calculations20,21 also indicate that this is the case. But until now, no structural model has been derived experimentally to confirm the endohedral nature of the metallofullerenes. Here we report the results of a synchrotron X-ray powder diffraction study of Y@C82 that confirms that the yttrium atom is located within the carbon cage. The yttrium atom is displaced from the centre of the C82 molecule and is strongly bound to the carbon cage.
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