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

Intercalation of C₇₀ with europium affords two kinds of magnetic compounds, a canted antiferromagnet Eu_xC₇₀ (X ≈ 3) and a ferromagnet E_xC₇₀ (x ≈ 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-_δC₇₀ (δ ≈ 0.27) is pseudo-monoclinic, derived by a simple deformation of the parent face-centered cubic (fcc) structure. Eu₉-_δC₇₀ (δ ≈ 0.2) forms an fcc structure, in which cuboctahedral clustering of Eu²⁺ ions is observed in the enhanced size octahedral holes. The observed T_c of the Eu₉-_δC₇₀ 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.