Fig. 1S.2. Dependence of Tc on Cs concentration for Cs;tRb3_iC60. The discontinuity in the dashed curve at x = 2.7 indicates a change in crystal structure from fee for x < 2.7 to bcc close to x = 3 [15.12],

(NH3)4Na cations [15.24], The remaining bare Na and Cs ions are found in tetrahedral sites [15.24], This is somewhat unusual, since in the pristine Na2CsC60 the Cs ion occupies an octahedral site while the Na ions preferentially occupy tetrahedral sites. The (NH3)4Na2CsC60 compound is analogous to ternary graphite intercalation compounds of the type M-C6H6 GICs, M-THF GICs, or M-NH3 GICs, where M denotes the alkali metal. Such compounds provided a powerful vehicle for tailoring the physical properties of GICs [15.25,26], Ternary fullerene compounds of this kind may likewise open up new possibilities for modifying the properties of fullerene host materials.

Since the addition of (NH3)4 to Na2CsC60 was effective in increasing Tc, ammonia was added to K3C60 in an attempt to enhance Tc further. A stoichiometry of (NH3)K3C60 was achieved upon NH3 addition, without additional charge transfer, but this compound was not found to be superconducting [15.27] even though the unit cell volume was almost the same as that for Rb2CsC60 with a Tc = 31.3 K (see Table 8.3). The introduction of (NH3) produced a 6% volume expansion of the unit cell, accompanied by a distortion of the crystal structure away from cubic symmetry. The absence of superconductivity in NH3K3C60 was attributed to either departures from a cubic crystal structure or charge localization on the Cg0 anions due to correlation effects [15.27].

Since the Tc of a superconductor such as KxRb3_xC6(l varies continuously with x [15.18], it is believed that the superconductivity in alkali metal alloy systems M3__XM'XC60 is not sensitive to superconducting fluctuations [15.20]. This is in contrast to organic superconductors such as (BEDT-TTF)2X, where a mixture of two different ions suppresses superconductivity [15.28,29],

It is generally believed that a relation between Tc and the electronic density of states at the Fermi level N(EF) is more fundamental than between Tc and the lattice constant a0, and efforts have been made to present the literature data in this form (see Fig. 15.3) [15.22], On the other hand, there is considerable uncertainty presently regarding the magnitude of the experimental electronic density of states for specific compounds (see ยง14.5), while the lattice constants can be more reliably measured. It is for this reason that plots of Tc vs. lattice constant a0 are more commonly used at the present time by workers in the field.

Superconductivity has also been observed in alkaline earth-doped superconductors with Ca, Ba, and Sr. Although the Tc value for these compounds is much lower than for alkali metal dopants, the superconductivity in these compounds is interesting in its own right and merits attention. Since the alkaline earth ions tend to have smaller ionic radii than the alkali metal ions (see Table 8.1), they would be expected to result in less lattice expansion

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