00q

Fig. 7.13. Phase diagram showing the various phases of C70, the possible phase transitions, and the stacking of the C70 molecules [7.96].

T01 = 340 K), the intermediate temperature regime (275 < T < 340 K), and the low temperature regime (below T02 = 275 K). We describe below the crystal structures that have been reported for each of these regimes. Although there is no agreement about the details of the structure in the three regimes, there is general agreement that there are three regimes, that the structure is isotropic above Tm and becomes more anisotropic as T is decreased. A compilation of physical parameters associated with solid C70 is shown in Table 7.12.

At high temperature (T 7q, ) the close-packed fee phase (a0 = 15.01 A) with freely rotating molecules is the most stable phase (see Table 7.12), but since the ideal hexagonal close packed (hep) phase with c/a0 = 1.63 is almost equally stable, fee crystals of C70 tend to be severely twinned and show many stacking faults [7.107]. In this connection we note that the stacking sequences of the close-packed planes follow an ABCABC sequence in the fee structure and an ABAB sequence for the hep structure. Although there is some disagreement among the various research groups about the crystal structure of the high-temperature phase (or phases), there is agreement that the structure shows little anisotropy, since the C70-C70 distances for the fee and ideal hep are essentially the same, except for the planar stacking along the (111) direction. NMR magic angle spinning experiments

Table 7.12

Physical constants for C70 solid materials.

Physical constants for C70 solid materials.

Table 7.12

Quantity

Value

Reference

fee lattice constant

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