G

Fig. 16.18. Measured EPR spectra for TDAE-C^ in the low-temperature region. "A" signifies the broad peak appearing below 16.7 K, whereas "B" denotes the narrow peak associated with Curie-Weiss-type behavior [16.95].

enhanced spin-spin correlations. This broad component increases in intensity as T decreases further below Tc. The intensity of the EPR line implies one unpaired electron per C60 unit [16.45,95], and the same result is obtained for TDAE-C70 [16.96], The relatively broad EPR line is attributed to the localization of the spins on the C6(l and other C~c anions. The detailed dependence of the linewidth on applied magnetic field is consistent with a field dependence of Tc [16.45], as discussed further in §18.5.2. The g-value of TDAE-Qo is between 2.0003 and 2.0008 at room temperature [16.95,100], close to the values reported for electrochemically prepared Cgo [16.78]. The high g-factor (2.0017-2.0025) below Tc for TDAE-C«, is attributed to the broad EPR line which appears below Tc. In contrast, the ^-factors for the TDAE-C„c compounds for nc — 70,84,90,96 (see Fig. 16.19) show little temperature dependence and are close to the free electron g-factor of 2.0023. The EPR linewidths for the higher fullerene derivatives also show only a weak temperature dependence, especially in comparison to that for TDAE-C60, as can be seen in Fig. 16.20.

The transition at Tc is also connected with a large 103) increase in the intensity of the ESR line for TDAE-C60 [16.45], indicative of a large increase in local correlations and in the magnetic susceptibility. Pulsed

Fig. 16.19. Experimental temperature dependence of the g-values of TDAE-fullerides CM, C70) C84, CM, C9 [16.95,96],

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