that support superconductivity in the alkaline earth-doped C60. The second relates to the divalence of the alkaline earth ions, where up to two electrons per dopant atom are donated to the C60-derived states. The tlu-derived band is filled at three alkaline earth dopants per C60 unit, and upon further metal ion addition, the /^-derived bands start to fill, reaching a metallic phase when they are about half-filled at a stoichiometry between five and six alkaline earth ions per C60 unit (see §14.1.3), since the metal-derived bands significantly hybridize with the C50 bands above the ¿¡„-derived band.
Figure 15.5 shows that the Tc for the fee superconducting alkaline earth Ca5Cfi0 follows the same relation between Tc and the C60-C60 separation that is found for the alkali metal M3C60 compounds, whereas the body-centered cubic (bcc) alkaline earth compounds Sr6C60 and Ba6C60 also show correlation between Tc and the C60-C60 distance, but show a different functional relation relative to the alkali metal compounds.
Regarding the issue of crystal structures, superconductivity in the alkali metal M^M^Qo compounds corresponds to the fee Fm3m space group, while superconductivity in the alkaline earth compounds corresponds to the following structures: Ca5C60 has a simple cubic structure where each of the four C60 anions of the fee structure is distinct, while Ba6C60 and Sr6C60 both have the bcc Im3 crystal structure (which is also the structure exhibited by the semiconductors KeQo and Rb6C60) (see §7.1.1 and §8.5). Since the superconductivity for all of these compounds is similar, we conclude that the superconductivity is predominantly influenced by the C60-derived states and to a lesser degree by the dopant species. With regard to band filling, the observation of superconductivity in Ca5C60, Ba6C60, and Sr6C60 indicates a transfer of electron charge carriers to C60, beyond the six electrons needed to fully occupy the tlu bands. These observations indicate that superconductivity is possible for both a half-filled tlu band and a partially (perhaps half) filled t]g band. Symmetry arguments (see §14.2.2) [15.30] suggest that if the
Fig. 15.5. Plot of the supercon-
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