© Copyright 1998 by the American Chemical Society
VOLUME 102, NUMBER 39, SEPTEMBER 24, 1998
LETTERS Dielectric Properties of C70-Solvate Crystals Grown from a Benzene Solution A. Sherman,† A. Talyzin,*,‡ M. El Gholabzouri,† P. Lunkenheimer,§ R. Brand,§ and A. Loidl§ A.F. Ioffe Physico-Technical Institute RAN, Politechnic str 26, 194021 St-Petersburg, Russia, Ångstro¨ m Laboratory, Inorganic Chemistry, Box 538, SE-751 21 Uppsala, Sweden, and UniVersity of Augsburg, Experimentalphysik V, UniVersity str.2, 86135 Augsburg, Germany ReceiVed: May 4, 1998; In Final Form: June 26, 1998
C70 crystals with size up to 3 × 3 × 0.2 mm were grown from benzene solution. The real and imaginary parts of the complex conductivity of this crystals were studied in the frequency range 20 Hz-1 MHz and the temperature range from 10 K to the room temperature. The temperature dependence of the real and imaginary parts of conductivity showed two frequency-independent anomalies at 150 and 275 K. These anomalies are interpreted as phase transitions.
1. Intoduction techniques1
Fullerenes can be grown by vapor or from solutions in organic solvents.2 C70 crystals up to 1 mm in size were grown by the vapor method in a temperature range of 700800 K.3,4 It is known that at high temperatures C70 grows as a mixture of fcc and hcp phases. The sequence of phase transitions upon cooling is complicated and not yet understood. Different researches reported on different phase transition temperatures. The most probable sequence of transitions is given by5
fcc rhombohedral monoclinic 340 K 275 K A similar sequence was found by Christides et al.6 in X-ray diffraction studies. Ramasesha et al.7 reported three phase transitions on cooling:
fcc hcpI hcpII monoclinic 340 K 325 K 275 K The existence of three phase transitions is also shown by Swarowsky et al.8 in a calorimetric study of C70. Single-crystal †
A.F. Ioffe Physico-Technical Institute RAN. Ångstro¨m Laboratory. § University of Augsburg. ‡
studies of the phase transition around 340 K carried out by Mitsuki et al.9 showed a strong hysteresis: on heating the transition from hcpI (a ) 10.1 A, c ) 18.5 A) to hcpII (a ) 10.5 A and c ) 17.3 A) appeared at 340 K, and on cooling, at 305 K. It is now clear that several phases can exist in solid C70, depending on sample preparation conditions. It should be noted that most of observed phases were obtained during cooling of crystals that were grown at high-temperature conditions. The only monoclinic10 and hcpII11 phases were reported to be grown at room temperature but under conditions far from equilibrium. Only very few data are available about C70 phases formed by crystallization from different solutions. It is known that C60 forms solvated crystals in a number of different organic solvents.12 It is also established that under certain conditions it is possible to grow pure C60 crystals from solution. Only Agafonov et al.13 reported C70 solvates obtained by evaporation of toluene solution at room temperature. The authors suggested a C70:toluene solvate of 1:1 composition and orthorhombic structure. On the other hand, some authors11 reported a pure C70 hcp structure obtained by evaporation of toluene solution at 110 °C. These authors suggested a difference in the growth temperature in order to explain why they did not obtain the solvated structure found in ref 13. Even less is known about the behavior of C70 in other
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Figure 1. X-ray powder patterns for C70 samples: (1) starting powder provided by MER corporation, (2) crushed crystals grown from the benzene solution.
solvents. Large C70 crystals were obtained by crystallization from benzene solution.14 No structural analysis was performed on these crystals. It is rather questionable if these crystals are pure or solvated C70 compounds. They appear to be stable even after several hours of heating at T ) 473 °C, while solvates reported for toluene are destroyed at 380-410 K owing to the loss of the solvent. It is of general interest to study the properties of crystals grown at room temperature from solution in organic solvents and to study phase transformations in such crystals in order to compare them with properties of crystals grown from vapor phase. The crystals of C70 grown from vapor usually are of small size (maximally 1 mm), while our crystals grown from solution are large enough and more convenient to perform complex studies of various structure-sensitive properties as, for example, the complex dielectric constant * ) ′ + i′′. To our knowledge, there have been only two dielectric measurements on C70 reported up to now.15,16 Both studies were performed on powder samples. Here we present the study of the dielectric properties of single-crystalline samples of C70 in a broad frequency range and for temperatures from 10 K to room temperature, where the real part of conductivity Ω ) 0′′ωS/d and capacity (which is proportional imaginary part of conductivity) C ) 0′S/d are measured. Here 0 is the absolute dielectric constant, ω is the angle frequency, S is the square of the electrode, and d is the thickness of the sample. 2. Experiments All experiments were performed on single crystals that were synthesized by using 99%-purified C70 powder purchased from MER corporation. The C70 crystals were grown by slow evaporation (1-2 mL per day) of benzene solution as was described before (ref 17). This method allows one to use a small amount of solvent (10-15 mL) for the growth of crystals of several millimeters in size. Crystallization can be performed at different temperatures without loss of the solvent and pollution of the environment. Crystals with the shape of flat rectangular platelets and sizes up to 3 × 3 × 0.2 mm were obtained at 35 °C. Crushed large crystals were used for powder X-ray diffraction analysis using a Siemens D5000 diffractometer. Silicon was used as internal standard in all experiments. The resulting spectra are shown in Figure 1. All reflections can be described by an orthorhombic structure with cell parameters a ) 20.99 Å, b ) 32.85 Å, and c ) 11.01 Å with a cell volume V ) 7596 Å3.
Figure 2. Temperature dependencies of complex dielectric function of C70 single crystals: conductivity (a, top) and capacity (b, bottom); solid symbols, cooling; open symbols, heating. For clarity, the inset in Figure 2b shows the behavior of the dependencies vs direction of temperature change.
These results are close to the data of Agafonov et al.13 (a ) 21.075 Å, b ) 32.99 Å, c ) 10.84 Å, and V ) 7540 Å3) for toluene solvate. The detail structural investigations of our crystals are reported in our previous paper.18 The thermogravimetrical experiments made on our sample showed a sharp anomaly at 150 °C with a mass loss of 1012% again close to the results of Agafonov et al.13 Taking into account these similarities, we suggest the same 1:1 composition of C70 to benzene for our crystals. The crystals appear to be stable even for several months of exposition. The same powder spectra were obtained for powder samples immediately after the growth procedure and after 4 months of exposition under ambient conditions. The real and imaginary parts of the complex conductivity were recorded in the frequency range 20 Hz-1 MHz using the autobalance bridge HP4284A. To carry out temperaturedependent measurements, the samples have been mounted in a helium refrigerator system. The samples had a typical size of 3 × 3 × 0.2 mm3. Electrodes for dielectric measurements were painted on the main face of platelets with silver paint. 3. Results and Discussion Figure 2 shows representative measurements of the temperature and frequency dependencies of the complex conductivity of C70. The temperature dependence of the real part of the conductivity (Figure 2a) shows two frequency-independent
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anomalies at 150 K (T1) and 275 K (T2). At the same temperatures these frequency-independent anomalies can also be be detected in the temperature dependence of the capacity (Figure 2b). These anomalies can be understood as phase transitions, but there are some special features that should be noticed. The dependence obtained upon cooling is not reproduced upon heating. The typical cooling curve of the capacity exhibits a jump down at 300 K, a small maximum near 275 K, and a linear rise in the temperature region between 225 and 150 K. At temperatures below 150 K the capacity is nearly constant. Upon heating the capacity is reproduced up to the T1 anomaly, but the jump between 150 and 225 K appears to be much stronger than the jump appearing upon heating. A strong hysteresis is also observed in the temperature region near 300 K. The dependence has a downward jump during heating, while on cooling we observed a downward jump also. A similar hysteresis was observed in the temperature dependence of the real part of the conductivity as well. Two distinct maxima can be observed at 275 and 150 K on cooling. The heating and the cooling curves are identical up to 200 K. The 275 K anomaly of the heating curve is lower than the maximum observed during cooling, and close to 300 K the heating curve goes down in contradiction with the behavior observed on the cooling curve. The anomalies T1 and T2 can be interpreted as phase transitions. As stated above, there are a number of publications on structural transformations in C70 crystals. The character and temperature of these transformations are essentially dependent on the crystal growth procedure and on the purity of source material. However, in the case of reasonably perfect single crystals that were synthesized from well-purified C70 powder by sublimation the most often cited phase diagram can be presented as
fcc rhombohedral monoclinic 340 K 275 K Evidently the phase transition from the rhombohedral to the monoclinic structure at 275 K is the most reliably established feature of the phase diagram for this compound. It is interesting to note that the anomaly of complex conductivity (complex dielectric constant) we have detected at T2 coincides closely with this transition temperature. As stated above the roomtemperature symmetry of our solvated crystals is orthorhombic, and we suggest a transformation to monoclinic cell at this temperature similar to pure C70. It should be noted that a similar anomaly was found in ref 16 where dielectric measurements were made on pure C70 powder samples. Concerning the second dielectric anomaly that we observed at 150 K, there is only one report on a probable lattice anomaly near this temperature according to Raman spectroscopy data.14 It should be emphasized that the authors of ref 14 indicated that they did not observe any differences of Raman spectra between samples made by the evaporation from benzene solutions or by sublimation methods. The nature of the strong hysteresis that we found during the cooling-heating cycles is not completely clear. It is known19 that C70 crystals often reveal stacking faults and sometimes are completely destroyed after passing through the phase transition owing to internal stresses. We suggest that our samples show similar behavior. Microscopic studies reveal that the crystals have a layered structure and these layers may split during the cooling-heating cycle.
Thus dielectric anomalies observed in the present studies are in good agreement with published data of the phase transformations in the pure C70 structure. It is remarkable that phase transition temperatures in our crystals, which contain about 10% of solvent, appear to be the same as found for pure C70. The density of our crystals calculated from X-ray data is 1.61 g/cm3, which is only slightly smaller than the density of pure fcc C70 (1.65 g/cm3). It shows that the structure of this solvated phase could be pictured as a slightly deformed C70 structure. The nature of these phase transitions is one of the most interesting problems. According to the reported data20 at present, the space group for the low-temperature monoclinic phase is not clearly identified, with C2, Cm, P2, Pm being the most likely candidates. As far as these groups are polar we can expect that the phase transition from the orthorhombic to the monoclinic phase can show features typical for phase transitions from nonpolar to polar, probably ferroelectric-like phases as well. Since dipolar anomalies were discovered in the region of the phase transitions, it is essential to discuss shortly the character of these anomalies and to compare them with similar anomalies at structural phase transitions. If we do not take the temperature hysteresis of the complex conductivity into account, both anomalies (at 275 and 150 K) are seen in the temperature dependence of capacity and real part of conductivity. The Ω(T) dependence clearly shows the existence of maxima at these temperatures, while the C(T) curves exhibit a change of slope at the same temperatures. Moreover, at temperatures just above T1 (150-230 K) and T2 (275-300 K at least on the heating curve), the observed anomalies have a linear decrease with increasing temperature. In principle there is a number of characteristic dielectric anomalies at structural phase transitions in different crystals depending on the type of transition. In particular there is a steplike temperature dependence of C at the transition to an improper ferroelectric (IF) (see for example ref 20). When our results are compared with characteristic types of dielectric anomalies at different types of phase transitions, it is apparent that the shape of the capacity anomaly in our crystals is similar to the anomalies of C at phase transitions to an IF. However, in the present case C reveals a linear increase with temperature above the transition temperatures T1 and T2 (Figure 2,b), instead of a jump21 as the temperature approaches the phase transition. As a probable reason for the smearing out of this jump, we suggest inhomogenities of our sample due to stacking faults or multitwinning. But overall, the observed C anomaly resembles the anomaly of capacity (dielectric constant) of the phase transition characteristic for improper ferroelectrics. It is also necessary to take into account that polar space groups were proposed for low-temperature phases of C70.20 The suggestion of an improper ferroelectric nature of the low-temperature phase of our crystals requires further experiments investigating the spontaneous polarization. 4. Conclusions (a) The first studies of the complex conductivity of large single crystals of C70 grown from a benzene solution were performed in the temperature range 15 K < T < 300 K. (b) Two anomalies were found at T1 ) 150 and T2 ) 275 K. (c) The anomaly at 275 K appears to be similar to the phase transition known for C70 grown from vapor. It shows that the solvent incorporated into the structure only slightly deforms the structure of pure C70.
7514 J. Phys. Chem. B, Vol. 102, No. 39, 1998 (d) The dielectric anomaly at 150 K coincides with the lattice anomaly known from Raman spectroscopy14 on crystals made by both evaporation of solution and sublimation methods. It is suggested that this anomaly is due to a phase transition similar to the T2 transition. To understand the nature of the possible phase transitions at the temperatures T1 and T2, more detailed structural, dielectric, and electric field investigations of synthesized crystals should be carried out. In particular, the search for a possible spontaneous polarization would be of great importance. Acknowledgment. This work is supported in part by 9702-18205 Grant RFBR and Russian Science Program “Fullerenes and Atomic Clusters” Grant N98 065. References and Notes (1) Meng, R. L.; Ramirez, D.; Jiang, X.; Chow, P. C.; Diaz, C.; Matsuishi, K.; Moss, S. C.; Hor, P. H.; Chu, C. W. Appl. Phys. Lett. 1991, 59, 3402. (2) Yosida, Y.; Arai, T.; Suematsu, H. Appl. Phys. Lett. 1992, 61, 1043. (3) Li, J.; Mitsuki, T.; Ozawa, M.; Horiuchi, H.; Kishio, K.; Kitazawa, K.; Kikuchi, K.; Achiba, Y. J. Cryst. Growth 1994, 143, 58-65. (4) Jiang, L.; Li, J.; Nagahara, L. A.; Kino, N.; Kitazava, K.; Iosida, T.; Hashimoto, K.; Fujishima, A. Appl. Phys. A 1995, 61, 17. (5) Vaughnan, G. B. M.; Heiney, P. A.; Fischer, J. E.; Luzzi, D. E.; Ricketts-Foot, D. A.; McGhie, A. R.; Yiu-Wing-Hui; Smith, A. L.; Cox, D. E.; Romanow, W. J.; Allen, B. H.; Coustel, N.; McCauley, J. P.; Smith, A. M., III. Science, 1991, 254, 1350.
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