© Copyright 1997 by the American Chemical Society
VOLUME 101, NUMBER 47, NOVEMBER 20, 1997
LETTERS Phase Transition C60-C60*4C6H6 in Liquid Benzene Alexandr V. Talyzin Institute of Earth Sciences, Uppsala UniVersity, NorbyVagen 18 B, 752 36 Uppsala, Sweden ReceiVed: June 20, 1997; In Final Form: September 8, 1997X
The results of the first direct observations of the transition between solvated C60*C6H6 and pure fcc C60 in benzene solution with optical microscopy and combined with X-ray powder diffraction are presented. It is shown that the solvated C60*4C6H6 structure at room temperature is significantly different from the lowtemperature structure described before. The temperature of phase transition was found to be between 40 and 41.5 °C. A new method of growth of solvated crystals based on recrystallization of face-centered cubic C60 to solvated form is described.
1. Introduction It is known that C60 forms solvated crystals in a number of different organic solvents.1 Ruoff et al.2 reported that the temperature dependence of solubility has its maximum near room temperature for several solvents. Smith et al.3 showed that this anomalous behavior of solubility can be explained thermodynamically. According to their model, two phases can exist in equilibrium with solution, one of them with a positive enthalpy of dissolution, another with a negative one. These phases can both be solvated with different amounts of solvent, or one of them can be free of solvent. Usually at room temperatures the solvated structure is stable in the solution. At some temperature a phase transition between the two forms occurs. The solvated crystal melts, and another phase forms: pure C60 or C60 with a lesser amount of solvent. This phase transition provides a change in the slope of the temperature dependence of solubility. This model is confirmed by the results of investigations of the C60 crystallization from benzene.4 It was known that the crystallization at room temperatures produces solvated crystals C60*4C6H6,5,6 while pure fcc C60 grows at the temperature of benzene boiling.7 In our previous studies we defined the temperature of transition between these two phases as 36 °C. At T < 30 °C solvated crystals were grown, at T > 36 °C fcc crystals were formed, and at 30 < T < 36 °C the formation of some intermediate crystals was observed.4 Differential scanning calorimetry experiments made X
Abstract published in AdVance ACS Abstracts, November 1, 1997.
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for a number of solvents8-10 show that such transitions are detected as a maximuma in the heat capacity temperature dependence. The temperature of transition between solvated and nonsolvated forms of C60 in a benzene solution was determined by Olofsson et al.8 as 49 °C, which is 9 °C higher than the value defined from our crystallization experiments. It is important to measure the temperature of this phase transition by an independent method. It is questionable if the triclinic centrosymmetrical structure determined at low temperatures, 173 K5 and 104 K,6 is still valid at room temperatures. Benzene solvated crystal is optically active at room temperatures (He et al.11), but rotation of the plane of polarized light cannot exist in centrosymmetrical structure described at low temperatures. The first direct observations of the phase transition between solvated C60*4C6H6 and pure C60 phases in a benzene solution as detected by optical microscopy and by powder X-ray diffraction analysis are presented in this paper. 2. Experiments C60 from MER corporation (99.9%) and distilled benzene were used. Special samples were used for direct microscopy observations of processes in solution. Small cells made of glass were filled with solution and the source fullerene material, and then the cells were hermetically closed by silicate glue. Such cells are routinely used for powder X-ray experiments. Two cells filled with the solution were made: one with solvated © 1997 American Chemical Society
9680 J. Phys. Chem. B, Vol. 101, No. 47, 1997
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Figure 2. Temperature dependence of C60 X-ray powder spectra in benzene solution.
Figure 1. (a, top) C60 solvated crystals in benzene solution at room temperature. (b, bottom) The same sample heated to 50 °C. Solvated crystals are melted to pure fcc C60 phase. (1 mm scale).
crystals grown by evaporation of benzene at room-temperature conditions and a second with the commercial powder of fcc C60. In the first cell a rapid recrystallization was observed. After 12-14 h all the material of source crystals transformed into well-shaped crystals with shiny faces and near hexagonal habitus (Figure 1a). The reason for the recrystallization is that source crystals were stored several days in air and partly lost their original solvated structure. Such crystals show only peaks of fcc structure on powder diffraction spectra even after short exposure to air. After such crystals were placed in benzene solution, new crystallites formed and then, due to difference in chemical potentials, C60 material moves from unstable fcc grains to solvated crystals stable at such conditions. The second cell shows a different result. At room temperature the process of transformation from fcc to solvated crystals was not visually observable for several days. Then this cell was cooled to 7 °C. The first small transparent crystals of red color appeared the next day, but even at this temperature recrystallization to solvated crystals continued several days. After 10 days, all source powder transformed to solvated crystals. It is remarkable that the cell recrystallization resulted in the formation of only a few crystals; the largest is more than 1 mm long, even when only 1-2 mg of C60 material are used. This allows the possibility of growing large crystals of the solvated structure by a very simple procedure of recrystallization. A usual commercial powder of C60 can be used as a starting material. The main advantages of this method are: (a) very small amounts of solvent are required, and (b) no evaporation of organic solvents is needed. To check this suggestion, one simple growth experiment was made. A small closed vessel with 30 mg of C60 and filled by benzene was placed in a
refrigerator with a temperature of 7 °C. After 10 days several needles with a size of up to 3 mm grew on the bottom of the vessel. According to the model of Smith et al.,3 the transition from a solvated to unsolvated form is an incongruent melting. The purpose of the following experiment was to examine how the process of transformation from one phase to another occurs. The first cell, containing only solvated crystals, was heated to 50 °C. After 10 min of heating at this temperature, dramatic changes were observed. Some of the solvated crystals were totally destroyed to black opaque powder, which consisted of small octahedral shaped crystallites typical for fcc C60. After 15 min all crystals of the solvated phase melted (Figure 1b). The process of transformation was also recorded by a video camera connected to the microscope. In the next step the cell was cooled to room temperature. Results were the same as observed in the second cell: there was no change at room temperature during several days followed by slow recrystallization to solvated crystals when the temperature was lowered to at 7 °C. X-ray powder diffraction methods were used for characterization of phases C60 and C60*4C6H6 and for defining a transition temperature between these phases. A cell with solvated crystals C60*4C6H6 was prepared as described above: fcc C60 powder stored in a benzene solution at T ) 7 °C for 2 weeks completely recrystallized to solvated crystals. X-ray diffraction data were obtained using a Siemens system consisting of a Smart CCD Area Detector and a direct-drive rotating anode as X-ray generator. Mo(KR) radiation (tube voltage 50 kV, tube current 24 mA, cathode gun 0.1 × 1 mm), monochromatized by using an incident beam graphite monochromator, was passed through a collimator of 0.5 mm diameter to the sample. The diffracted X-rays were collected on a 512 × 512 pixels area detector. This method provided measurements of a large portion of the Debye ring at different 2θ settings. Good results were obtained despite a relatively large size of crystallites in the sample, which resulted in a number of diffraction spots on the Debye image. Diffraction spectra were recorded during a slow heating of the cell starting from room temperature. The usual time of X-ray exposure at each step was 300 s. The results are shown in Figure 2. The spectra recorded at room temperature definitely do not belong to fcc C60. But it appears that it cannot be
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J. Phys. Chem. B, Vol. 101, No. 47, 1997 9681
Figure 3. X-ray powder spectra of solvated C60 at room temperature. Positions of peaks calculated from the data of Balch et al.6 are shown by vertical lines.
described as triclinic, as reported before5,6 (cell parameters: a ) 9.99 Å, b ) 15.072 Å, c ) 17.502 Å, R ) 65.26°, β ) 88.36°, γ)74.94°). Heating of the sample showed no changes below 40 °C. At T ) 41.5° slow changes occur. The three spectra made at this temperature are shown in the Figure 2: the first recorded immediately, the second after 30 min, and the third after 1 h of exposure at 41.5 °C. Some changes are easily recognizable. The number of spots due to the diffraction from relatively large crystallites is decreased on the Debye image. This proves that the destruction of the solvated structure takes place already at this temperature, but the process is slow. Then the temperature was increased to 47 °C, and changes appeared to be very rapid. After 40 min of heating at this temperature peaks of solvated structure were totally replaced by peaks of fcc C60. Records made after cooling of the cell to 24 °C (Figure 2) showed that the reverse transformation to the solvate phase is very slow, as slow as it was during the direct microscopy observation. 3. Discussion al.,3
solvated crystals According to the model of Smith et C60*4C6H6 are in equilibrium with the solution, while the fcc C60 phase cannot be stable at room temperatures. Our experiments confirmed this model: the temperature of phase transition defined by X-ray powder diffraction methods for first the time is between 40 and 41.5 °C. This value is in good agreement with data obtained in previous crystallization experiments.4 X-ray diffraction spectra of the benzene solvated phase appear to be in poor agreement with the structure reported before.5,6 Crystals in the sample have some preferent orientation, because it was studied “as grown”. To obtain a more representative spectra, several images were collected from different points of the sample and then integrated as one. The resultant spectra are shown in Figure 3. Positions of peaks calculated from the data of Balch et al.6 are shown by vertical lines. The only lowangles part of the spectra is shown where the difference is seen more clearly. There are not less than two peaks (2θ ) 2.06, 3.86), which cannot be described by the structure proposed by Balch et al.6 The total number of registered reflections is not enough for structure determination; the structure has low symmetry and is most probably triclinic like the structure described before. In the present case, measurements were made directly in solution with the phase completely stable at these conditions. Meidine et al.5 and Balch et al.6 made structural analyses at low temperatures (173 and 104 K, respectively). The final
structure definition of the room-temperature structure should be done by single-crystal methods on a wet sample, the usual practice for solvated crystals that are not stable out of solution. A probable temperature of transformation from the lowtemperature solvated structure to the room-temperature structure may be the temperature of benzene freezing, 5.5 °C. According to data of He et al.,11 the room-temperature structure is not centrosymmetrical, unlike the low-temperature structure. The model of Smith et al.3 is purely thermodynamic. This theory explains well the unusual temperature dependence of C60 in benzene but cannot predict the temperature of transition between solvated and unsolvated forms of C60. The probable explanation of why this phase transition occurs at the temperature 40-41.5 °C is some change of local structure of pure liquid benzene, which has been detected by Raman spectroscopy12 and by the presence of a heat capacity anomaly13 within the temperature region 37-40 °C. This temperature is in perfect agreement with the previous data obtained by crystallization experiments4 and with data reported in this paper. The behavior of solubility and heat capacity similar to the system C60/benzene is known for a number of other solvents.8,9 It appears that each particular solvent forms its own solvates with C60 and has its own temperatures of transition between solvated and nonsolvated forms of C60. These phase transitions may also correspond to some weak changes in the local structure of solvents, which has an influence on the stability of phases. In summary, the report of first direct visual observations of the phase transition from solvated to pure C60 in benzene solution combined with X-ray powder diffraction measurements is presented here. The temperature of the phase transition was found to be between 40 and 41.5 °C. It is shown that the structure of the solvated form of C60 at room condition is different from that at low temperature, which was reported before. A method of growth of solvated crystals based on recrystallization from fcc C60 to C60*4C6H6 at temperatures below the point of phase transition is presented. References and Notes (1) Smith, A. L.; Li, D; King, B.; Zimmerman, G.; In Recent AdVances in the Chemistry and Physics of Fullerenes and Related Materials, 1994; Electrochemical Society Proceeding Series, Pennington, NJ, 1994; PV 9424, p 443. (2) Ruoff, R. S.; Malhotra, R.; Huestus, D. L.; Tse, D. S.; Lorents, D. C. Nature 1993, 362, 140. (3) Smith, A. L.; Walter, E.; Korobov, M. V.; Gurvich, O. L. J. Phys. Chem. 1996, 100, 6775. (4) Talyzin, A. V.; Ratnicov, V. V.; Syrnicov P. P. Phys. Solid State 1996, 7, 1531. (5) Meidine, M. F.; Hitchcock, P. B.; Kroto, H. W.; Taylor, R.; Walton, D. R. M. J. Chem. Soc., Chem. Commun. 1992, 1534. (6) Balch, A. M.; Lee, W. J.; Olmsted, M. M. J. Chem. Soc., Chem. Commun. 1993, 56. (7) Yosida, Y. ; Arai, T.; Suematsu, H. Appl. Phys. Lett. 1992, 61, 1043. (8) Olofsson, G.; Wadso, I.; Ruoff, R. S. In Recent AdVances in the Chemistry and Physics of Fullerenes and Related Materials, 1996; Electrochemical Society Proceeding Series, Pennington, NJ, 1996; PV 9610, p 17. (9) Korobov, M. V.; Mirakyan, A. L.; Avramenko, N. V.; Odinec, I. L.; Ruoff, R. S. In Recent AdVances in the Chemistry and Physics of Fullerenes and Related Materials, 1996; Electrochemical Society Proceeding Series, Pennington, NJ, 1996; PV 96-10, p 5. (10) Smith, A. L.; Strawhecker, K.; Shirazi, H.; Olofsson, G.; Wadso, I.; Qvarnstrom, E. In Recent AdVances in the Chemistry and Physics of Fullerenes and Related Materials, 1997; Electrochemical Society Proceeding Series, Pennington, NJ, 1997, in press. (11) He, H.; Barras, J.; Foulkes, J.; Klinowski, J. J. Phys. Chem. B 1997, 101, 117. (12) Pinan-Lucarre, J. P.; Loisel, J.;. Berreby, L; Dayan E.; Dervil E. J. Raman Spectrosc. 1992, 23, 67. (13) Rozhdestvenskaya, N. B.; Smirnova, L. V. JETF Lett. 1985, 44, 165.
