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electron-rich C=C double bond. This effect contributes to orienting the C60 molecules and also restrains their rotational degrees of freedom [8.7]. Thus K3C 60 and RbjQo do not have rapidly rotating molecular anions at room temperature, as do the undoped C60 molecules. Finally, a repulsive interaction between the alkali metal cations and the C60 anions due to the size of the metal cations [see Fig. 8.6(b)] is important, especially for the larger ions (K+ and Rb+), tending to orient the C^, anions so that the hexagonal faces are adjacent to the alkali metal ions. Because the repulsive metal-C60 interaction is large for K3C60 and Rb3C60, their crystal structures differ from that for the Na^C^ compounds, where this contribution to the orientational potential is much less important (see §8.5.3).

The crystal structure resulting from consideration of orientational alignment effects in the case where the repulsive interaction shown in Fig. 8.6(b) is important is illustrated in Fig. 8.7. In this figure the CM anions are seen to align so that twofold axes of the C60 anions are along x,y, and z axes. As discussed in §7.1.2, the point group Th is a subgroup of Ih so that the alignment for each molecule as shown in Fig. 8.7 is consistent with the highest-symmetry alignment of the C60 molecules in the fee basic crystal

Fig. 8.7. Crystal structure of the K3C60 and Rb3C60 compounds. The open and dark spheres represent the alkali atom at tetrahedral and octahedral sites, respectively. Note the two possible orientations of the C60 molecules. Site locations for tetrahedral and octahedral sites relative to the origin (center of lower left C«, anion) and the cubic lattice constant for ^Qo are given [8.107].

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