at T = 261 K

d Intensity of fee forbidden x-ray peaks (4,5,0) and (4,5,1) [7.56,88],

d Intensity of fee forbidden x-ray peaks (4,5,0) and (4,5,1) [7.56,88],

The temperature dependence of the order parameter Xl may be compared with that of x-ray diffraction spots such as (4,5,1) [7.88] and (4,5,0) [7.56], which are forbidden in the fee structure. The (4,5,1) [or (4,5,0)] peak intensity decreases from its value at very low temperature and experiences a discontinuous jump of about half of its intensity at T = 261 K (see Table 7.11) [7.88]. The T dependence of this x-ray peak is similar to the T dependence for Xx shown in Fig. 7.12. A discontinuity in the order parameter is observed in the low-energy region of the Raman spectrum (~30 cm-1) [7.94], and it is noted that the observed temperature dependences of the libron intensity and the uncorrelated Lorentzian scattering intensity are similar to the temperature dependences of X2 and X3, respectively.

In accordance with the discontinuous change from ratchet motion to relatively free molecular motion, the lattice should expand as a result of surmounting the very weak van der Waals interaction, as the weak Coulomb interaction between electron-rich double-bond regions on one molecule with electron-poor pentagon face regions on the adjacent molecule (see Fig. 7.7) become less effective when the rapid spinning motion commences. Weak hysteresis effects near Tm are characteristic of a first-order transition in which there is a local maximum in the free energy between two local minima as a function of the order parameter. As stated above, the basic driving force behind the phase transition at T01 is the competition between entropy gain by rotation and energy gain by intermolecular attraction [7.92],

7.2. Crystalline C70 and Higher-Mass Fullerenes

The crystal structure of C70 is more complex than that of C60 [7.86,95-98], evolving through several distinct crystal structures as a function of temperature (see Fig. 7.13), including the high temperature regime (above

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