"All superconducting compounds in this table crystallize in the fee structure (except for CsjCflj), neglecting the orientation of molecules with respect to crystal axes (see text). However, the compounds LiK2C60, Li2KC60, and NaK2C60 are multiphase, and KCs2C60 is unstable [8.115],
''The octahedral and tetrahedral site symmetries are denoted by O and T, respectively. cThe Tc for Cs3C60 increases under application of pressure, with a maximum Tc of ~40 K achieved at ~12 kbar [8.117]. However, the crystal structure of the superconducting phase for Cs3C60 is not believed to be fee [8.117].
dOrthorhombic space group P2221, which is distorted from the fee structure common to M^so compounds. The value of 14.52 A given in the table is an average of the a, b, c lattice constants of the orthorhombic structure, a = 14.971 A, b = 14.895 A, and c = 13.687 A [8.120],
As discussed in §7.1.3, various interactions determine the orientation of the C60 anions in crystal structures, the most important being an orienta-tional potential consisting of a van der Waals attractive force between the C60 anions and a short-range Lennard-Jones repulsive interaction, as shown in Fig. 8.6(a), where a potential minimum is seen at ~23°, corresponding to the orientation of C60 molecules in the Pa3 structure (see §7.1.3). For alkali metal-doped compounds, two additional alkali metal-C60 anion interactions must be considered. Shown in Fig. 8.6(c) is the contribution to the orientational potential from the attractive electrostatic alkali metal-C60 anion force, which tends to orient the C60 anions to lie close to an
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