Journal of the American Chemical Society, Vol.125, No.7, 1897-1904, 2003
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 EuxC70 (x approximate to 3) and a ferromagnet EuxC70 (x approximate to 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 Eu2+ for both systems. The structure of Eu3-deltaC70 (delta approximate to 0.27) is pseudo-monoclinic, derived by a simple deformation of the parent face-centered cubic (fcc) structure. Eu9-deltaC70 (delta approximate to 0.2) forms an fee structure, in which cuboctahedral clustering of Eu2+ ions is observed in the enhanced size octahedral holes. The observed T-C of the Eu9-deltaC70 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 Eu2+-Eu2+ distances and high coordination numbers permitted by the multiple occupation by Eu2+ ions of the expanded octahedral interstitial sites in higher fullerene-based solids.