DETERMINATION OF THE ENDOHEDRAL NATURE OF THE METALLOFULLERENE Y@C₈₂ BY MEM. M. Takata, E. Nishibori, B.Umeda, M. Sakata, M.Ohno, H. Shinohara & Y. Saito* , Nagoya University, Nagoya 464-01, Japan.,*Mie University, Tsu 514 Japan

The first conclusive evidence of endohedral nature of the Metallofullerene Y@C₈₂ has been obtained via a Synchrotron X-ray powder diffraction study using the Maximum Entropy Method(MEM). Recently, the synthesis of fullerenes encapsulating various metal atoms within the carbon cage (endohedral metallofullerenes) has stimulated wide interest because of their unusual structural and electronic properties. Observations using STM, EXAFS, HRTEM and ESR have strongly suggested that the metal atoms are indeed inside the fullerene cage. Theoretical calculations 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. The Y@C₈₂ fullerene was separated and isolated by the two-stage high performance liquid chromatography(HPLC) method. The purity of the Y@C₈₂ fullerene was more than 99.9%. An X-ray powder pattern of Y@C₈₂ was measured by using Imaging Plate at Photon Factory BL-6A₂. The wavelength of incident X-rays is 1.0Å. The space group is assigned to P2₁, monoclinic. The experimental data were analyzed in an iterative way of combination of Rietveld analysis and the MEM. The reliable factor of the obtained MEM charge density is 1.4%. In the MEM charge density, there exist remarkably high densities just inside the C₈₂ cage. The number of electrons around the maxima is about 38 which is very close to the atomic number of a yttrium atom. Evidently, the density maxima at the interior of the C₈₂ cage is the yttrium atom. The present study revealed the fact that the yttrium atom is displaced from the centre of the C₈₂ molecule and is strongly bound to the carbon cage.