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INTRODUCTION Many properties of Buckminsterfullerene, C60, have been extensively studied as functions of both temperature, T, and pressure, p. The crystal structure and dynamics are now well known as functions of T at normal pressurel, 2 and the high-p phase diagram has also been carefully mapped near and above room temperature 3. However, few studies have been carried out under pressure at low T, and little information is available on phases and structures in this range. Compressibility data 4 for C60 have recently been obtained at this laboratory at temperatures down to 152 K, and combining those data with literature data for the structure 5 under pressure we have predicted 6 the existence of an orientationally ordered phase at sufficiently high pressure. Although the actual crystal structure(s) of a material can only be found by x-ray or neutron diffraction experiments, the phase diagram can often be mapped more easily by finding the phase boundaries by other means. In particular, measurements of the thermal or electrical transport properties are often used for this purpose, since these properties are continuous functions of T and/or p within each phase but often change more or less discontinuously on going from one crystal phase to another. Such measurements can also provide information on structural order or disorder. We have therefore measured for the first time the thermal conductivity X of C60 at high pressure to obtain more information on the phase diagram and, indirectly, on the lattice structure and dynamics. From the measured data we find the p dependence of the glass transition temperature Tg and also a very interesting and unexpected relaxation behaviour near T9 under pressure. EXPERIMENTAL DETAILS We used the hot-wire method 7 to measure simultaneously X and the heat capacity per unit volume, pcp, where p is the density. The probe was a 0.1 mm in diameter Ni wire, surrounded by the material investigated and heated by short power pulses. X and pcp were then obtained by fitting a theoretical expression to the measured data for T vs. time. The estimated inaccuracy in X, * Permanent

address: Institute for Low Temperature Physics and Engineering, Ukrainian Academy of Sciences, Kharkov, Ukraine. 549 Mat. Res. Soc. Symp. Proc. Vol. 359 0 1995 Materials Research Society

was less than ±2% above 200 K but increased to ±4% at 40 K. The C60 specimen was identical to that used in our recent compressibility study4 . It was supplied by Term USA, Berkley, CA, and had a stated purity >99.9%. The sample was loaded into the pressure cell in dry argon and the cell was then mounted in a piston-cylinder device described elsewhere 8 . The temperature was varied by cooling the pressure vessel with a closed cycle helium refrigerator and p was calculated as load/area with an empirical correction for friction. EXPERIMENTAL RESULTS AND DISCUSSION Figure 1 shows our experimental data for X as a function of T at several pressures in the range 0.1 to 1 GPa. Except at 0.3 GPa, the data shown were obtained during cooling of the s