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Thermodynamic Characterlzatlon of C60by Differential Scanning Calorimetry7 Yimin Jin, Jinlong Cheng, Manika Varma-Nair, Guanghe Liang, Yigang Fu, Bernhard Wuaderlich,* Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 -61 97, and Department of Chemistry, The University of Tennessee, Knoxville, Tennessee 37996- 1600
Xiao-Dong Xiang, Roman Mostovoy, and Alex K. Zettl Materials Science Division, Lawrence Berkeley National Laboratory and Department of Physics, The University of California, Berkeley, California 94720 (Received: December 23, 1991; In Final Form: February 18, 1992)
Heat capacity measurements on Csowere carried out by differential scanning calorimetry from 120 to 560 K. This heat capacity was linked based on existing normal-mode and lattice calculation, to the vibrational heat capacities. In this way the thermodynamicfunctions are available from absolute zero of temperature. The phase transition at 256 K could be identified as a crystal-to-plastic crystal transition with an increase in entropy of 27.3 J/(K mol). A broad beginning of this transition at 190 K is linked to the change from a jumplike rotation that is starting already at about 100 K without increase in entropy to the rotational motion that ultimately causes the disordering transition. This classification is based on reinterpretation of the solid-stateNMR results with calorimetry data. No melting temperature could be detected up to 950 K, the temperature limit of our calorimeter.
I. Iatroductioa Besides diamond and graphite, the two well-known crystalline allotropes of carbon, a totally new form of crystalline carbon has been discovered a few years ag0.I It consists of ball-shaped Cso molecules (buckminsterfullerene) and has become well-known because of its special structure? Before 1990 most research on Cso was limited to theoretical studies3-I2because of insufficient material to undertake large-scale experimentalstudies. In 1990, Kriitschmer et al. first reported a method to synthesize C6,, in macroscopic quantity.I3J4 Since then, the study of this new class of compounds and their derivatives is receiving more a t t e n t i ~ n . ' ~ -For ' ~ example, there has been a characterizationof the molecular structure using mass s p e c t r ~ s c o p y ' * ~ * "and ~ J ~X-ray , ~ ~ diffractionI4 as well as UV-vis light,7.I3,17.18 IR 13,14,I7-19 Raman,20 NMR, 17.1 821-23 and BRIT spectroscopies and high-pressure liquid chromatography (HPLC).17 tR*lcnted at the American Physical Society Meeting, March 16-20, 1992.
Initial information about a phase transition below room temperature became available recently through differential thermal analysis.2e26 Coupled with X-ray data, orientational ordering was suggested to occur on cooling. In this paper measurements and computations of heat capacity will be reported. The computations are based on vibrational spectra. These new data permit a quantitative interpretation of the transition and, in addition, allow the generation of complete thermodynamic functions, of enthalpy (H), entropy (S), and Gibbs function (G). The heat capacity of Cso was measured from 120 to 560 K by using a single-run differential scanning calorimetry (DSC) t e ~ h n i q u e ~which ~ - ~ ~needs only 20-30-mg samples to produce quality data. The heat capacity contribution from vibrations was calculated next, based on existing normal-mode and lattice vibration frequen~ies,"~*~~q~~ and then compared with the experimental data. It will be shown that the phase transition that occurs at 256 K is a crystal-to-plastic crystal transition with an entropy of 27.3 J/(K mol), typical for such transitions involving orientational disordering. A broad beginning of this transition can be
0022-365419212096-5151%03.00/0 0 1992 American Chemical Society
5152 The Journal of Physical Chemistry, Vol. 96, No. 12, 1992
linked to the change in motion from jumplike rotation, initiated already in the crystal at much lower temperature, to isotropic rotation. This interpretation is based on the known solid-state NMR ~ p e c t r u m and ~ ~ -confirmed, ~ in this paper, by heat capacity investigations. Finally, an effort to establish the melting temperature was limited to the observation that no melting occurred up to 950 K, the upper range of our DSC. 11. Experimental Section
Sample. Fullerenes were extracted from fullerene-rich soot (Texas Fullerenes, Inc.) using a Soxhlet extractor with benzene or hexane. The extracted c 6 0 was separated from C70 and the higher fullerenes using an alumina chromatographic column. The Ca powder was then washed with ether and dried in vacuum at about 475 K. The purity of the sample was better than 99.5%, checked by mass and Raman spectroscopies. calorimetry. A commercial Thermal Analyst 2100 system from TA Instruments Inc. with a 912 dual-sample DSC and DSC autosampler were used for heat capacity measurements. Details about the equipment, operation, and software development were given in previous publication^.^^-^^ A single-run heat capacity measurement technique was used.35 Heat capacities were determined from 120 to 560 K with four sets of experiments (120-220, 180-320, 310-420, and 380-500 K). Each run was repeated five times, and the collected data were averaged and smoothed. The error is estimated to be
