Direct EELS observation of the oxidation states of Sm atoms in Sm@C2n metallofullerenes (74⩽2n⩽84)

The oxidation states of Sm atoms in various fullerene cages are directly investigated by electron energy-loss spectroscopy. The observed peak positions of the M45 edges of the Sm atom in Sm@C2n [2n=74, 78, 80, 82 (isomers I, III), 84 (isomers I, II, III)] are apparently shifted to the lower binding energy region in comparison with those of a trivalent Sm3+ in Sm2O3. The results indicate that the Sm atoms take +2 valence states irrespective of the fullerene cages. This observation is discussed based on a simple thermochemical cycle model. We also report that the Sm metallofullerenes have been converted to Sm carbide materials under a prolonged electron beam irradiation.