Jul 1, 1992 - Professors G. Chouteau, A. Dworkin, J.E. Fischer and J-P. Michenaud for enligthing discussions. This work is part of the program âAction de ...
Solid State Communications, Vol. 85, No. 2, pp. 73-75, 1993. Printed in Great Britain.
0038-1098/93$6.00+.00 Pergamon Press Ltd
(Received 1 July 1992, acceptedforpublication 23 October 1992 by D. Van Dyck)
The specific heat of fullerenes extract (c(jo/c70) has been measured in the temperature range 4 c T < 560 K. The temperature variation of the specific heat exhibits an anomalous behaviour. Below 80 K, it presents a classical temperature variation with a dependence close to a 13 below 10 K. The measurements bring two transitions to the fore : one around 250 K corresponding to the well known orientational phase transition, and the other around 90 K which is attributed to the freezing of the rotational motion of the molecules.
approximately 80% C60, 18% C7u and 2% of residual solvent. A parallelipedic sample (25 x 5 x 0.8 mm3) was messed under 8 kbar durine 5 minutes. The as-obtained material was hard and b&e and had a specific mass of 1.45 g cm-3. From this parallelipiped, a sample of 70 mg was cut for the specific heat measurements. For the measurements below room temperature (4 c T c 300 K), we used a system based on the relaxation time method* 2. Above room temperature (300 < T < 560 K), the specific heat was measured by differential scanning calorimetry on a Perkin-Elmer DSC-2 system. The temperature variation of the specific heat of the fullerenes extract is presented in figure 1. For comparison, we have also plotted on this figure the specific heat of graphitet3. It may be seen that the room temnerature value of -the specific heat is approximately equal to 700 mJ g-tK-t. At high temperature, the specific heat of CdC7u is the same as that of graphite as expected from the Dulong et Petit law. Below 260 K, it becomes slightly higher than that of graphite but follows almost the same temperature variation. Below 80 K, it decreases less ranidlv than the specific heat of graphite. So, it is clear hat: at lower temperatures, fullerenes present a large excess of specific heat with respect to graphite. Indeed, at 10 K, the specific heat of CjdC70 is more than an order of magnitude larger than that of graphite. The same difference as been observed by Beyernrann et al.14 for pure Cm. This large difference mav be attributed to two phenomena, both acting simultaneously. On the one hand, the freezing of a substantial orientational disorder at low temperatures induces an excess of entropy in the material. This may lead to an excess of specific heat. On the other hand, the low frequency lattice vibrational modes and the librational modes observed in this materiall5,Ie may contribute to create the excess of specific heat of fullerenes with respect to graphite. Where the temperature range of measurements overlap, the peculiar temperature dependence observed is in agreement with that reported by Atake et al.11 for pure Ceu. It reflects the different aspects of the structural modifications occuring in the material as the temperature varies.
Since the discovery by Kriitschmer et al.1 of an efficient synthesis method for the production of C61-j buckminsterfullerenes. this new form of carbon has stimulated significant scientiEc interest. At room temperature, solid Cen presents a facecentered-cubic (f.c.c.) crystal structure with the Ceu molecules orientationallv disordeted2.3. This structure corresponds to a cubic-close-packed arrangement of Crju molecules which are uncorrelated with their neighbours due to rapid reorientational motions4. The existence of a first order transition between 250 and 260 K has been observed by differential scanning calorimetry (DSC)5 and by X-ray diffraction3. This transition is due to a reorientational ordering of the C6u molecules which leads to a simple-cubic (s.c.) crystal structure at low tempemtures3~6~7. This orientational order is attributed to Van der Waals bonding and to electrostatic interaction between electron-poor regions (pentagonal faces) and electron-rich regions (inter-pentagon double bounds) of adjacent molecules6~7~s. This first-order phase transition leads to an abrupt decrease of the lattice parametet@ and to a sharp increase of the thermal conductivitv of Cr;ntu. It also leads to the observation of a large anomaly in-the specific heat between 180 and 260 K”. Neutron diffraction7 and thermal conductivity measurementstu also revealed the existence of a kind of second-order transition around 90 K. This transition is attributed to the freezing of some orientational disorder at low temperatures. It is related to the existence of a second preferred orientation due to the Van der Waals interactions (double bonds facing hexagonal faces) with an energy slightly higher than the orientation previously described and separated from this ground state by a large energy barrier (- 260 meV)7,*,10. In this study, we have measured the specific heat of a mixture of fullerenes (C&C_ru) between 4 and 560 K. We will show how its temperature dependence reflects the different structural aspects of the material described above. The fullerenes extract was obtained from Texas Fullerenes Corporation, TX, USA. The composition was * to whom all correspondence should be addressed. 73
FULLERENES EXTRACT
74
10-11”“”
100 10 Temperature [K]
1000
Figure 1: Temperature dependence of the specific heat of fullerenes extract (C&70) compared to that of graphite14 (full line). It is possible to distinguish between two regions in the temperature dependence of the specific heat. Below 80 K, the specific heat seems to follow a classical behaviour. At low temperature (T < 10 K), the temperature variation tends to a T3 law. When the temperature increases, the specific heat gradually saturates. In order to estimate the Debye temperature, we tried to fit the temperature dependence below 9 K to a 13 law (Fig. 2). The best lit was obtained when an additional linear term The Debye temperature was then was introduced. estimated to be of the order of 190 K. This value is much higher than that obtained by Beyermann et a1.14. This is probably due to the fact that they applied a more complete In their model for a larger temperature range. measurements and in their analysis, they also observed the existence of a linear term in the low temperature specific heat of C& This linear term cannot be attributed to an electronic contribution since the material is an electrical insulator. However, it could reveal the existence of disorderinduced localized states14 similar to those which are - 25 T Y ‘; ol20
Vol. 85, No. 2
responsible for the low-temperature linear specific heat of amorphous solidsl7. A possible candidate for these twolevels systems are the two nearly degenerated orientations, if we assume that molecules can move from one orientation to the other by phonon assisted tunneling. Above 85 K, the specific heat starts again to increase. In figure 3, we present the details of the temperature variation between 50 and 300 K. Two anomalies may be observed : a first one between 220 and 260 K consisting in a double hump and a second one around 90 K. The first anomaly is certainly due to the first-order phase transition associated with the onset of the molecules free rotation. The fact that the hump is not as large as the one observed on pure Cfj~)~~may be attributed to the presence of C70 in the mixed fullerenes extract . It is also certainly related to the poor crystallinity of our material, since we did not make any heat treatment to remove the residual solvent. Indeed, it has been shown that the transition temperature and the value of the enthalpy are very sensitive to the purit and the crystallinity of the material18 and to its origintdl However, the presence of a double humo cannot be attributed to the presence of C7o since it has also been observed on oure Gr?n. It is oossible that it is also due to the poor cry&llinity-of the material and to the presence of residual solvent. The second anomaly observed around 90 K is probably related to the second-order transition due to the freezing of the orientational disorder7.10. It is possible that the presence of higher fullerenes (mainly C70) and of residual solvent has somehow affected the behaviour of the specific heat and has damped the anomalies due to the phase transitions occuring in Cm. However, we think that our measurements of the specific heat of mixed fullerenes extract reveals the main thermal characteritics of this material and confirms the results of the previous analysis performed by means of different techniques.
Acknowledgment - The authors wish to thank the Unit6 de Physique et de Chimie des Hams Polymeres (UCL) for the use of their DSC equipment. They ate also very grateful to Professors G. Chouteau, A. Dworkin, J.E. Fischer and J-P. Michenaud for enligthing discussions. This work is part of the program “Action de Recherche ConcerGe” sponsored by the Belgian State (Ministry of Scientific Policy).
I”“,‘,“,““,““. :
C,=TT+aT3
:
T=893pJ9_‘K-*
2 z 15 e g 10 100 i!L u) 5
4
5
6
7
6
150 200 250 Temperature [K]
300
9
Temperature [K]
Figure 2 : Detail of the specific heat variation below 9 K showing the fit used to estimate the Debye temperature and which reveals the existence of a linear term.
Figure 3 : Detail of the temperature variation of the specific heat between 50 and 300 K showing the anomaly observed between 220 and 260 K corresponding to the first-order transition and that observed around 90 K attributed to orientational-disorder freezing.
Vol.85, No. 2
FULLERENESEXTRACT
75
REFERENCES
1 W. Kr&schmer, D.L. Lamb, K. Fostiropoulos and D.R. Huffman, Nature 347,354 (1990) 2 R.M. Fleming, T. Siegrist, P.M. Marsh, B. Hessen, A.R. Kortan, D.W. Murphy, R.C. Haddon, R. Tycko, G. Dabbagh, A.M. Mujsce, M.L. Kaplan and S.M. Zahurak. Materials Research Society Symposium
Proceedings ~01206 : Clusters and Clusters-Assembled Materials. ed. R.S. Averback, J. Bemholc & D.L.
Nelson (MRS. Pittsburgh, 1991), p 691 3 P.A. Heiney, J.E. Fischer, A.R. McGhie, W.J. Romanow, A.M. Denenstein, J.P. McCauley and A.B. Smith III. Phvs. Rev. Letters 66.2911 (1991) 4 C.S. Yannoni, ‘RD. Johnson, G. Meijer, D.S. Bethune and J.R. Salem, J. Phys. Chem 959 (1991); R. Tycko, R.C. Haddon, G. Dabbagh, S.H. Glarum, D.C. Douglass and A.M. Mujsce, J. Phys. Chem. 95,518 (1991); R.D. Johnson, D.S. Bethune and C.S. Yannoni, Act. Chem Res. 25,169 (1992) 5 A. Dworkin. H. Szwarc. S. Leach. J.P. Hare. T.J. Dennis,‘H.W. Kro&R. Taylor and D.R.M. Walton, C. R. Acad Sci. Paris - Serie II 312,979 (1991) 6 W.I.F. David, R.M. Ibberson, J.C. Matthewman, K. Prassides, T.J.S. Dennis, J.P. Hare, H.W. Kroto, R. Taylor and D.R.M. Walton, Nature 353, 147 (1991) 7 W.I.F. David, R.M. Ibberson, T.J.S. Dennis, J.P. Hare and K. Prassides, Europhys. Letters 18,219(1992) 8 J.P. Lu, X.P. Li and R.M. Martin, Phys. Rev. Letters 68, 1551(1991)
9 P.A. Heiney, G.B.M. Vaughan, J.E. Fischer, N. Coustel, D.E.Cox, J.R.D.Copley,D.A. Neumann,
W.A. Kamitahara, K.M. Creegan, D.M. Cox, J.P. McCauley and A.B. Smith III, Phys. Rev. B 45, 4544 (1992) 10 R.C. Yu, N: Tea, M.B. Salamon, D. Lorents and R. Malhotra. Phvs. Rev. Letters 68.2050 (19921 11 T. Atake, T.‘Ta&ka, H. Kawaji, K: Kikuchi, K:Saito, S. Suzuki, I. Ikemoto and Y. Achiba, Physica C 185-189, 427 (1991) 12 H.D. Vasileff, J. Appl. Phys. 23,979 (1952); I.S. Kouvel, J. Appl. Phys. 27,639 (1956); R.L. Fagaly and R.G. Bohn, Rev. Sci. Instrum 48, 1502 (1977) 13 W. DeSorbo and W.W. Tyler, J. Chem Phys. 21, 1660 (1953): P.H. Keesom and N. Pearlman. Phvs. . Rev. 99. i ii9 (i955) l4 W.P. Bevermann. M.F. Hundlev. J.D. Thomoson. F.N. Diederich and G. Grtiner, Whys.Rev. Le~ters’68, 2046 (1992) 15 C. Coulombeau, H. Jobic, P. Bemier, C. Fabm, D. Schtltz and A. Rassat, J. Phys. Chem %, 22 (1992) 16 G. Chouteau, private communication 17 P.W. Anderson, B.I. Halperin and C.M. Varma, Phil. Msg. 25, 1 (1972) l8 A. Dworkin, C. Fabre, D. Schlitz, G. Kriza, R. CColin, H. Szwarc, P. Bemier, D. Jerome, S. Leach, A. Rassat, J.P. Hare, T.J. Dennis. H.W. Kroto. R. Tavlor and ’ R.R.M. Walton, C. R. ‘Acad. Sci. P&is - Skie II 313, 1017 (1991) 19 A. Dworkin, private communication 20 J.S. Tse, D.D. Klug, D.A. Wilkinson and Y.P. Handa, Chem. Phys. Letters 183,387 (1991)
