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[8.77,113]

"See also Table 8.3, where the lattice constants, symmetries, and superconducting transition temperatures Tc for M3C60 and related crystals are listed.

bFor the (bet) body-centered tetragonal structure (14/mmm), the in-plane and c-axis lattice constants are denoted by a and c, respectively.

cTo compare a[cc to abcc, use the relation abcc = afcc/V2.

dThe alkaline earth compounds Ca5C60, Ba6C60, Sr^C^ and the rare earth YbjQo are superconducting (see §15.1), with Tc values of 8.0 K, 6.2 K, 4.0 K, and 4.0 K, respectively.

stoichiometry M,C60 (see Table 7.2 for site symmetries). If both tetrahe-dral and octahedral sites are fully occupied, then the composition M3C60 is achieved [8.107]. Referring to Table 7.2, we see that the tetrahedral sites (c) and the octahedral sites (b) do not lower the symmetry of the space group (as discussed in §7.1.2) associated with C60 anions, provided that all the tetrahedral and all the octahedral sites are occupied [8.70,121,122], The space group describing the crystal structure for K3C60 and Rb3C60 has been given as Fm3m, the same structure as for C60 above Tm (see §7.1.1) [8.123]. The FmZm accurately describes the structure for K3C60 and Rb3C60 if orientational effects of the C60 molecules are ignored and the molecules are considered as spheres.

Table 8.3

Lattice constants and superconducting Tc values for alkali metal M3C60, C60, and related compounds."

Table 8.3

MjC«,

Dopant site sym.6

a (A)

Tc (K)

References

Qo

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