line earth dopant atom are transferred to the Cm molecule, due to hybridization of the tlg-derived band with the s band of the alkaline earth (see §12.7.4).

The behavior of Ba^Qo (see Fig. 14.5) has many similar features to SrrC 60 and CaxC6(), but also some differences, attributed to the larger size of the Ba2+ ion relative to the size of the tetrahedral vacancy, while the opposite is true for Ca2+ and Sr2+ (see Table 8.1). Thus, a stable phase for BaxCb0 is believed to occur at x ~ 1, where Ba2+ occupies the octahedral site [14.49] in the fee structure. At x = 3 where the ilu band is filled, Ba3C60 exhibits the A15 crystal structure, while the superconducting compound Ba6C60 exhibits the bcc crystal structure [14.52] (see §8.6.2). The resistivity at the composition Ba^go is the lowest thus far reported for an alkaline earth-doped fullerene solid (see Table 14.2).

Studies of the Ba^Qo system demonstrate the importance of high temperature annealing for achieving greater stoichiometric homogeneity and better crystallinity [14.49]. It is also of interest to see that for an annealing temperature of 220° C, metallic conduction (Ea = 0) is achieved over a wide range of x values near x = 5. The samples used for superconductivity studies (see §15.1) were at a stoichiometry Ba6C60 and were annealed well above 220°C [14.52],

It has also been reported that partial occupation of the ilg band could be achieved by observing photoemission spectra from C60 on potassium surfaces [14.53],


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