Alkaline Earth Metal Fullerides

In BajCgQ the band is completely filled. Higher doping levels are reached in the superconductors CajCgQ and Ba5C5Q. In these compounds, the band is also occupied with charge carriers, as shown by the valence band spectra [131, 138]. Therefore, their superconductivity is associated with the band. 

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Fullerenes can be easily chemically reduced by the reaction with electropositive metals [1, 97-99], for example, alkali- and alkaline earth metals. The anions Cjq"" (n = 1-5) can be generated in solution by titrating a suspension of in liquid ammonia with a solution of Rb in liquid ammonia [100], whereupon the resulting anions dissolve. Monitoring of this titration is possible by detecting the characteristic NIR absorption of each anion by UV/Vis/NIR spectroscopy. The solubility of the alkali metal fullerides in the polar solvent NHj demonstrates their salt character. 

Fig. 5 Various C60 fullerides with alkali and alkaline-earth metal atoms [21]...

The production of metal fullerides succeeds as well with alkaline earth metals and rare earth elements. Among others, there are mixed alkali/alkaline earth fullerides with a composition of M3, eBaxQo (0.2 < x < 2 M = K, Rb, Cs). Likewise may pure alkaline earth fullerides be obtained, for example, CasCGo that becomes a supraconductor below 8.4 K. Barium and strontium give binary fullerides too, but with stoichiometric factors of 3, 4, and 6. Ytterbium and samarium may intercalate in the C,so-crystal as well, yielding supraconductive compounds with a composition of M2.75C1S0. 

Determination of the superconducting properties of salts prepared from Ceo and alkali and alkaline-earth metals with relatively high transition temperatures (Tc 33 K) led to the observation that the temperature Tc for the superconductivity of these fullerides was related to the size of the metallic atoms and the lattice constant [71]. Consequently, the preparation of complexes with bigger organic donor molecules was thought to increase the Tc values and a number of charge-transfer complexes with a variety of donor molecules have been prepared [70]. 

After the production of crystalline solid C60> various C60 compounds with metallic elements called fullerides have been produced. In Fig. 5, alkali and alkaline-earth fullerides are summarized [21]. Since fee (sc) C60 has two tetrahedral (T) and one octahedral (O) interstitial sites per C60> there are various fee A3C60 fullerides, where A=Na, K, Rb, Cs or their combinations. Interestingly, most of them... 

The room-temperature conductivities of these compounds are usually about one or two orders of magnitude smaller than those shown in Table 12.2. Included in Table 12.3 are the superconducting (but very air-sensitive) alkali metal and alkaline earth fullerides these are compounds with three-dimensional superconductivity, where the alkali metal ions are just gegenions tucked in tetrahedral and octahedral holes in the cubic fullerene crystal structure [36]. The critical temperatures of ET salts seem to be stuck at... 

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