0 50 100 150 200 250 300

temperature (K)

Fig. 14.21. Temperature variation of the cubic lattice constant a0 for solid C^. The inset shows the low-temperature data including the anomaly at ~ 90 K on an expanded scale [14.167,168], and found to be about 80% of that for C60 prior to doping. The compressibilities for K3C60 and Rb3C60 are similar but slightly smaller than for C60 itself, corresponding to about 70% and 80% of the C60 compressibility, respectively [14.153,170],

14.11. Thermal Conductivity Because of the low carrier concentration in C60 and related compounds, the dominant contribution to the thermal conductivity k is due to lattice vibrations. No experimental information is yet available on the electronic contribution Ke to the thermal conductivity of C60. However, as shown theoretically by Gelfand and Lu [14.22], if there were an electronic contribution to k(T), one would expect the Wiedemann-Franz law kJctT — L to hold, where L is the Lorenz number. The lattice contribution given by the classical relation kl = |C„uA, where v is the velocity of sound and A is the phonon mean free path, suggests that the contribution kl should be small as well.

Results on the temperature dependence of the thermal conductivity k(T) have been reported [14.171] for a single-crystal sample of C6(l over the temperature range from 30 K to 300 K. These results show an anomaly in k(T) near 261 K associated with the first-order structural phase transition at Tm (see §7.1.3). The k(T) results further show the magnitude of the maximum thermal conductivity Kmax to be less than that for graphite (inplane) [14.172,173] and diamond [14.174] by more than three orders of magnitude. The low value of the thermal conductivity k is attributed to

60 80 100 temperature (K)

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