THERMAL-CONDUCTIVITY OF C-60 AND C-70 CRYSTALS

We have performed thermal conductivity measurements on C60 and C70 crystals grown by sublimation. For single crystal C60, the thermal conductivity kappa is approximately 0.4 W/m K at room temperature and is nearly temperature independent down to 260 K. We observed a sharp orientiational phase transition at 260K, indicated by a 25% jump in kappa. Below 90 K, kappa is time dependent, which manifests itself as a shoulder-like structure at approximately 85 K. The temperature and time dependence of kappa below 260 K can be described by a simple model which accounts for the thermally activated hopping of C60 molecules between two nearly degenerate orientations, separated by an energy barrier of approximately 240 meV. It is found that solvents have a strong influence on the physical properties of C70 crystals. For solvent-free C70 crystal, kappa is about constant above 300 K. There is a broad first-order phase transition in kappa, at 300 K with a 25% jump. We associate this transition with the aligning of the fivefold axes of the C70 molecules along the c-axis of the hexagonal lattice. Upon further cooling, kappa, increases and is time independent.