Preparation and properties of alkaline-earth and rare-earth fullerides.

Superconductivity with record high temperatures for organic compounds has been demonstrated in alkali-metal intercalated C-60 fullerite. Divalent alkaline-earth (AE) intercalants, Ca, Sr, Ba have also be intercalated in fullerite, to form new fulleride compounds and some of these compounds exhibit superconductivity too. Here, the divalent character, and the size of the dopant AE atoms manifests itself in a smaller intermolecular separation and a complex, hybrid electronic band structure. A strong cation-C-60 interaction locks C-60 molecules in orientationally ordered novel phases. The A15 phase of barium fulleride for example is exclusively stabilized by a strong cation-electron deficient five-membered-ring interaction. Rare-earth (RE) metal intercalation in fullerite have also been successful for Yb, Sm, and Eu, and a remarkable vacancy-ordered (RE)(2.75)C-60 phase is discovered, with divalent cations. Microwave absorption studies suggest a T-c = 12.5 K superconductivity, in LaxC60.