Thermophysical Properties of C 70 Up To 1 Gpa
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INTRODUCTION Most physical properties of C60 are now fairly well known1 as functions of temperature T, and the effects of pressure p on the structure and other properties have also been extensively studied such that the p-T phase diagram is known over quite wide ranges in p and T. A large variety of interesting structural and dynamic features have been observed and the study of C60 has turned out to be an unusually fruitful field in solid state physics and chemistry. However, the properties of other fullerenes are much less known. This is probably largely due to the fact that the higher fullerenes are much more difficult to produce in pure form, but it also seems that these materials show an even more complicated behaviour vs. T and p than C 60 . Taking the second
lightest fullerene, C70 , as an example, the true zero-pressure structural and dynamic properties are still hotly debated, and different groups have found different lattice structures and phase transition temperatures even for seemingly well defined, highly pure samples. We have recently measured the compressibility 2 and thermal conductivity 3 of nominally 99.9 % pure C60 up to about 1 GPa and at temperatures between 50 and 340 K. While the data obtained were interesting and important in themselves, an even more interesting result was that they enabled us to map the phase diagram in surprising detail over the p-T range investigated 4 . Using literature data for the orientational structure we could explain both the presence of anomalies in the compressibility and the absence of glass transition anomalies in the thermal conductivity in certain pressure ranges, as well as a surprisingly small bulk modulus in the s.c. phase. Having thus found that studies of thermophysical properties can give important information on the phase diagrams of fullerenes we have applied the same methods to C70 , and we present preliminary results below. The magnitudes of the bulk modulus and the thermal conductivity of C70 are similar to those for C60 , as might be expected. The thermal conductivity is quite sensitive to rotational disorder and we can thus map the p-T phase boundary for molecular rotation in C70 , which is believed to coincide with the transformation from a room-temperature phase, probably with a mainly rhombohedral structure, to a low-T phase with a lower symmetry. * Permanent address: Institute for Low Temperature Physics and Engineering, Ukrainian Academy of
Sciences, Kharkov, Ukraine. 555 Mat. Res. Soc. Symp. Proc. Vol. 359 0 1995 Materials Research Society
EXPERIMENTAL DETAILS The material investigated was obtained from Term USA Ltd, Berkeley, CA, and had a stated purity of 99.8% (referred to fullerene purity). The specimen was first heat treated at 650 K and 10-2 torr before the experiment to remove solvents, and a Raman test indicated less than 0.1% solvent in the sample used. About 2.2 g of powder was compacted around a Ni hot-wire probe in a toroidal groove in a Teflon pressure cell under an atmosphere of dry Ar, and the closed cell was mounted in a
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