7 [ /


• 1 nut

■r * • • *

1 10 Temperatur* (K)

Fig. 14.22. Thermal conductivity of (a) polycrystalline Cm/C10 compacts (filled circles and squares) compared with that of amorphous Si02 (open circles) and amorphous As2S3 (asterisks) and a T2 dependence (dashed line), (b) Carbon in its three phases: diamond (open diamonds), single-crystal Cm (solid line), and single-crystal graphite, heat flow in the ab-plane (filled stars) and parallel to c-axis (open stars). The dashed line corresponds to the thermal conductivity based on the Einstein model (Eq. 14.17). [14.141], carriers. In addition to this diffusion process, which gives rise to a diffusion term Sd in the thermopower, there is also a phonon drag contribution Sp arising from the flow of phonons from the hot to the cold junction, dragging carriers with them as they are transported. Thus we normally write the following expression for the temperature dependence of the thermopower:

The thermopower for undoped C60 and C70 is negligible because of the absence of carriers. Thus physical thermopower measurements of fullerenes have focused on M3C60 (M = K, Rb) and KXC70 (jc ~ 4).

The temperature dependence of the thermopower S(T) of single-crystal K3C60 and Rb3C60 (prepared from CS2 solution) has been measured from

300 K down to low T [14.37]. The results (see Fig. 14.23) show that S(T) is negative (consistent with conduction by electrons) and nearly linear in T for both K3C60 (above ~150 K) and Rb3C60 (above -100 K) [14.32,37,177]. The results have been used to obtain an estimate for the Fermi energy and bandwidth using the arguments given below.

100 200 Temperature (K)

100 200 Temperature (K)

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