Bridging fullerenes with metals

Geometric and electronic structures of metal-substitutional fullerene C59Sm and metal-exohedral fullerenes C60Sm

The geometric and electronic structures of metal-substituted fullerene C59Sm and exohedral fullerenes C60Sm are studied using the density-functional theory. The geometric optimization shows that the replacement of a C atom with a Sm in C60 yields a stable substitutionally doped fullerene C59Sm, and among the five possible optimized geometries for C60Sm, the most favorable exohedral sites are above the center of a hexagon and a pentagon ring. The calculations for electronic structures show that the magnetic moment of Sm is preserved for all the stable structures as tiny hybridization takes place between the orbitals of the Sm atom and those of their neighboring carbons. Because of the small energy gaps and the half occupation of the highest occupied molecular orbitals, all the stable C60Sm isomers are inferred to be conductors.

Synthesis, structure, and magnetic properties of the fullerene-based ferromagnets Eu3C70 and Eu9C70

Intercalation of C(70) with europium affords two kinds of magnetic compounds, a canted antiferromagnet Eu(x)C(70) (x approximately 3) and a ferromagnet Eu(x)C(70) (x approximately 9) with transition temperatures (T(C)) of 5 and 38 K, respectively. The Curie constants in the paramagnetic phase and the saturation moment in the ferromagnetic phase are both understood by the full moment of Eu(2+) for both systems. The structure of Eu(3)(-)(delta)C(70) (delta approximately 0.27) is pseudo-monoclinic, derived by a simple deformation of the parent face-centered cubic (fcc) structure. Eu(9)(-)(delta)C(70) (delta approximately 0.2) forms an fcc structure, in which cuboctahedral clustering of Eu(2+) ions is observed in the enhanced size octahedral holes. The observed T(C) of the Eu(9)(-)(delta)C(70) ferromagnet is comparable to or larger than those of simple binary Eu-based ferromagnets, such as Eu chalcogenides or carbides, despite the low atomic ratio of Eu in the chemical formulas. This can be understood by the short Eu(2+)-Eu(2+) distances and high coordination numbers permitted by the multiple occupation by Eu(2+) ions of the expanded octahedral interstitial sites in higher fullerene-based solids.