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Rotational Dynamics in C70: Temperature- and Pressure-Dependent Infrared Studies K. Thirunavukkuarasu,† V. C. Long,‡ J. L. Musfeldt,^,‡ F. Borondics,||,§ G. Klupp,§ K. Kamaras,*,§ and C. A. Kuntscher*,† †
Experimentalphysik II, Universit€at Augsburg, D-86159 Augsburg, Germany Department of Chemistry, State University of New York at Binghamton, Binghamton, New York 13902, United States § Research Institute for Solid State Physics and Optics, H-1525 Budapest, Hungary ‡
ABSTRACT: We revisit the infrared (IR) spectra of C70 on pure samples as a function of temperature (20-370 K) and pressure (010 GPa). Although the rotation of the molecule is not completely free at ambient conditions, the measured spectra are in perfect agreement with previous density functional theory (DFT) calculations on isolated molecules, including the intensities, for 22 out of the possible 31 IR-active modes. We assign the anomalies in the infrared spectra both on decreasing temperature (∼280 K) and increasing pressure (∼0.8 GPa) to the freezing out of uniaxial rotation; the boundary between uniaxial and quasi-free rotation has a much weaker effect. At ∼7.5 GPa an irreversible change of the spectrum occurs, which we attribute to room-temperature dimerization. The pressure at which this dimerization occurs adds a new point to the phase diagram presented by Sundqvist.
’ INTRODUCTION Despite being the second most abundant fullerene, the phase diagram of C70 has been investigated only scarcely. This is mainly due to the fact that its crystal structure is far more complicated than that of C60: it exhibits a variety of phases depending on temperature, pressure, and thermal history, and it may be even impossible to produce phase-pure crystals. At ambient conditions, the morphology of C70 crystals can be face-centered cubic (fcc) or hexagonal close-packed (hcp). Most of the latter are in fact mixtures of fcc and rhombohedral (rh).1 On the basis of anomalies in various physical properties, two phase transition temperatures are generally agreed on: around 280 and 340 K.2 The fcc phase is found to be stable above 350 K, but at lower temperatures the structure can usually be described as a mixture of coexisting fcc, rhombohedral, and monoclinic phases. The monoclinic phase has not been observed above 280 K. Structural changes have also been investigated as a function of pressure. Most studies agree on a transition pressure at room temperature between 0.7 and 1 GPa3-6 and attribute it to an fccto-rhombohedral transition. However, because of the possible coexistence of phases mentioned above, the phase transition does not happen abruptly. Christides et al., based on their energy dispersive X-ray diffraction studies,6 suggested that the onset of the fcc-rh transition occurs around 0.35 GPa, with both phases coexisting up to 1 GPa at room temperature. Generally, the compression of the lattice forces the C70 molecules to line up r 2011 American Chemical Society
parallel, inducing and enhancing the orientational ordering of the molecules that leads to a rh symmetry of the lattice.7 Thermal conductivity and compressibility studies on C70 found two anomalies at around 0.15 and 0.7 GPa at 296 K.8-10 These anomalies were explained by a continuous change of the orientational ordering and eventual “freezing” of molecular rotations probably accompanied by a fcc-rh structural phase transition in analogy to the changes observed upon decreasing temperature. The knowledge gathered hitherto is summarized in the review by Sundqvist.7 He presents a “rotational phase diagram” based on the consideration that the multitude of phases can be arranged in a systematic way determined by the rotational state on the molecular level. On decreasing volume (decreasing temperature or increasing pressure) the molecular motions freeze in the order free rotation f uniaxial rotation f static. On the two ends of the scale, a perfectly ordered static system would be monoclinic, and a crystal with perfectly freely rotating molecules would be fcc; in reality, a slight orientational disorder is enough to change the former to rhombohedral and the latter to hcp.1 In addition, supercooling can easily occur, and in most cases several phases coexist. This situation is almost impossible to handle by methods which detect long-range order (e.g., diffraction studies). However, when using vibrational spectroscopy as a probe, we can take advantage of the fact that in molecular crystals the Received: January 3, 2011 Revised: January 27, 2011 Published: February 16, 2011 3646
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The Journal of Physical Chemistry C spectra are determined rather by the properties of the molecule than by the long-range order of the environment. At room temperature, where the fullerenes are rotating rapidly, the spectrum in the solid state is identical to that in solution. Although it is known that C70 samples can differ in their crystallographic properties, all published C70 infrared spectra are identical regardless of the crystal structure. As we know from C60, during an orientational phase transition the motion of the molecules stops; thus, the rotation no longer averages out the symmetry, and the molecule is locked in a fixed position in the crystal. Since the 5-fold axis, one of the principal symmetry elements of the fullerenes, is incompatible with any crystallographic point group, this freezing of the rotation inevitably means reduction of molecular symmetry, resulting in spectral changes. As long as the molecules are rotating, they are not sensitive to the stacking details. When applying pressure, the compression of the lattice favors the rhombohedral structure with the molecules lined up parallel.7 The rhombohedral and monoclinic structures differ only slightly, and the long-range order can be considered merely a perturbation compared to the rotational state of the molecule. Raman and infrared spectroscopic studies under pressure on C70 at room temperature indeed showed such effects. Some of the earlier investigations used mixtures of C60 and C70 which is undesirable as the presence of C60 influences the physical properties of C70.11,12 In pure C70, Sood et al. found an anomaly at around 1 GPa in the pressure dependence of the frequencies and line widths of the Raman-active vibrational modes, which was attributed to the fcc-rh orientational ordering transition.13 Maksimov and co-workers reported two anomalies at around 2 and 5.5 GPa, assigned to the features related to the orientational ordering of C70 molecules.14,15 In contrast, infrared measurements on pure C70 found no discontinuous change in the pressure dependence of the vibrational modes.16 Sundqvist proposed the p-T phase diagram of solid C70 based on these experimental investigations.7 From the phase diagram it is obvious that the pressure dependence of C70, especially for pressures above ∼3 GPa, needs to be further investigated. At higher temperature and pressure, C70 can also undergo polymerization, albeit less readily than C60. The possible polymer phases and their properties have been described in a second review by Sundqvist.17 In order to follow the transitions on a molecular level, we have performed infrared absorption measurements on the same C70 samples as a function of both temperature and pressure. Our results prove that the governing parameter is the rotational state of the molecules, regardless of the overall crystal structure. The most striking change in the spectra occurs at 280 K at ambient pressure and at 0.7 GPa at ambient temperature and can be identified as the stopping of the uniaxial rotation (the molecular driving force behind the fcc-rhombohedral phase transition). Thus, despite the lack of long-range order in most crystals, the temperature- and pressure-dependent behavior can be explained by molecular properties. In addition, we found an irreversible change above 7 GPa and assign this to the formation of C140 dimers.
’ EXPERIMENTAL METHODS C70 was purchased from Term USA (>99% purity) and used without further purification. The most common contaminant in C70 is C60, which can be easily detected from the infrared spectra. In our case, C60 vibrations were below the detection level even in
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the most concentrated samples; therefore, we regard the material as pure for our purposes. Earlier X-ray diffraction results of Soldatov and Sundqvist10 showed this type of sample to contain fcc and other unidentified phases. For the temperature-dependent measurements, we used several FTIR instruments (Bruker IFS 28, BOMEM DA and Bruker IFS 113v) with mercury cadmium telluride and Si bolometer detectors. We recorded the spectra from 50 to 700 cm-1 in polyethylene pellets and those from 400 to 2000 cm-1 in KBr pellets, with 0.3 and 1 cm-1 spectral resolution, respectively. We cooled the pellets in a liquid helium flow-through cryostat and took the data during heating from the base temperature in order to avoid supercooling of the samples. The pressure-dependent transmittance measurements were performed at room temperature using a Bruker IFS 66v/S spectrometer coupled with an infrared microscope (Bruker IRscopeII). A diamond anvil cell (DAC) of Syassen-Holzapfel type18 was used for generation of pressure, and the ruby luminescence method was used for pressure determination.19 The transmission of a mixture of powder sample and the pressure transmitting medium was measured for pressures up to 10 GPa over the frequency range 40020 000 cm-1. The measurements were performed with a resolution 1-2 cm-1 in the mid-infrared and 8 cm-1 in the near-infrared and visible frequency ranges. The quasi-hydrostatic pressure transmitting medium used in the all measured frequency ranges was CsI. In order to determine the transmittance of the fullerene C70 under pressure, the intensity Is(ω) of the radiation transmitted by the mixture of the powder sample and the pressure transmitting medium was measured. As reference, the intensity Ir(ω) transmitted by the pure pressure transmitting medium inside the DAC was used. The transmittance was then calculated according to T(ω) = Is(ω)/Ir(ω). For all measurements, absorbance is defined as A = log(1/T).
’ VIBRATIONAL MODES Ambient Conditions: Concentration Dependence and Comparison with Calculations. C70 has 204 vibrational de-
grees of freedom. The symmetry of the isolated molecule is D5h, where the vibrational modes can be described as follows: ΓðD5h Þ ¼ 12A 1 0 x 9A 2 0 x 9A 1 00 x 10A 2 00 x 21E1 0 x 22E2 0 x 19E1 00 x 20E2 00
ð1Þ
The ten nondegenerate A200 and the 21 doubly degenerate E10 modes are infrared-active. In the solid state, if the rotational motion averages the crystal field effects, the vibrational spectrum does not differ from that of the isolated molecule. This is true both for “free” rotation and uniaxial rotation because the latter happens around the principal C5 axis, thus preserving the D5h point group. Below the orientational transition, however, the symmetry reduction becomes apparent, and splitting of the E10 modes as well as activation of silent modes can occur. Further splitting (Davydov splitting) is caused by the presence of four molecules in the unit cell in the monoclinic phase. A factor group analysis (Z = 4, site symmetry Cs)23 predicts 408 IR-active vibrations at low temperature. The same number of Ramanactive modes is expected, but fewer have been found experimentally23 due to accidental degeneracies and very small splitting, which yielded rather a broad structure than individual peaks, even at high resolution. 3647
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Table 1. Frequencies of the Vibrational Modes (in cm-1) of C70 at Temperatures 77, 298, and 335 K, Compared to DFT Calculations of Sun and Kertesz20 and Stratmann, Scuseria, and Frisch21 77 K
298 K
335 K
phase III
phase II
phase I
a
323 361
360
360
418 458 (rh), 460
458
458
510
calcd ν
calcd int
calcd ν
calcd int
label
ref 20
ref 20
ref 21
ref 21
A200 E10 E10 E10 A200
318
0.2
318
0.2
326
0.1
326
0.1
358
0.6
359
0.6
414
0.3
416
0.3
459
11.2
459
11.2
E10
507
0.2
507
0.2
535
534
534
E10
533
34.5
533
33.7
564
564
564
A200
564
11.7
564
11.6
568, 578
578
578
E10
573
24.9
573
24.9
642, 643
642
642
E10
639
13.1
639
12.9
674
674
674
E10
665
22.1
667
22.8
A200 E10
704 729
0 0
703 730
0 0
E10
751
18.1
751
8.3
E10
828
0
828
0.1
696 728, 742,
Figure 1. Left panel: temperature dependence of the spectrum of C70 in the FIR frequency range (polyethylene pellets, resolution 0.3 cm-1). Right panel: temperature dependence of the spectrum of C70 in the MIR frequency range (KBr pellets, resolution 1 cm-1). Three typical temperatures are displayed for the three phases. The mode at 1385 cm-1, marked by an asterisk, is a vibrational mode of KBr which appears at low temperature.
745 795
795
795
898
897
A200
896
0.5
896
0.5
905
905
E10
905
0.3
906
0.3
1088
1087
1086
E10
1087
4
1088
3.9
1127, 1134
1133
1132
A200
1143
14.2
1144
4.2
E10
1177
0.6
1178
0.6
1176 1205
1204
1203
1206
1251,
1251,
1251,
E10
1255
1.3
1256
1.2
1258
1258
1.7
1258
1293
1292
1291
E10
1290
2.5
1291
2.5
1321
1321
1320
E10
1318
1.8
1319
1.9
1325 1415,
1324 1414
1323 1413
A200 E10
1321 1415
2.2 15.8
1321 1416
2.3 6.8
1429,
1429
E10
1431
198.9
1432
198.4
1460
1460
A200
1462
8.9
1463
8.9
E10
1489
8.4
1491
8.5
1560 1585
1559 1585
A200 E10
1568 1569
0.8 3.9
1567 1569
0.7 3.7
1422 1428, 1431 1446,
1431
1461 1562 1585 a
From ref 22.
First we investigated the infrared spectra in detail. To this end, we obtained the far- and mid-infrared spectra of a series of polyethylene and KBr pellets, respectively, with different concentration at room temperature. This procedure was necessary in order to detect the lowest intensity modes, although under certain conditions, the highest intensity features then became saturated. The observed IR-active modes are summarized in Table 1 along with the DFT calculations of Sun and Kertesz20 and Stratmann, Scuseria, and Frisch.21 The agreement, even with the intensities, is extremely good. Even in the most concentrated
Figure 2. Temperature dependence of selected portions of the infrared spectrum of C70. Left panel: polyethylene pellet, 0.3 cm-1 resolution; right panel: KBr pellet, 1 cm-1 resolution. Three typical temperatures are displayed for the three phases. Asterisks indicate weak modes appearing in phase III.
polyethylene pellets, we did not observe the lowest frequency weak far-infrared mode at 323 cm-1 reported by Lebedkin et al.22 in thick pellets of the neat material, but we include it in Table 1 for the sake of completeness. Otherwise, our results agree with previously published studies.22,24-26 Even though it is highly improbable that all these samples had the same long-range crystal structure, the reproducibility of the spectra proves that they are determined by the molecular structure. Temperature Dependence. Table 1 includes the frequencies of the modes at three temperatures, typical of the three known phases based on consensus concerning the transition temperatures. Following Sundqvist,7 we call these phases subsequently phase I, II, and III, in order of decreasing temperature. As described in the Introduction, the molecules in phase III are static, in phase II they rotate around the C5 axis, and in phase I they perform free rotation. Overall spectra of the three phases are depicted in Figure 1 and selected regions in Figure 2. The changes with temperature are small but significant when going from phase III to phase II, and there is no observable change when going to phase I. In phase III, the vibrational mode at 458 cm-1 exhibits a clear splitting (left panel of Figure 2). For the analysis of the splittings, we chose a vibration in the far-infrared part, where we could measure with high enough resolution so that splittings could be 3648
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Figure 3. Temperature dependence of the parameters of the 458 cm-1 mode of C70: wavenumber (ν*), integrated intensity (A), and width (w). Dashed lines denote the transition temperatures between the phases which are also indicated. Figure 6. Pressure-dependent frequencies of selected vibrational modes of C70 for pressure up to 7 GPa.
Figure 4. Comparison of the spectrum at 0.2 GPa in the pressure cell with that at room temperature in a KBr pellet. Apart from the interference fringes of the DAC, the two look identical, proving that at this low pressure no phase transition occurred.
Figure 7. Pressure derivative of the frequency of some vibrational modes as a function of pressure. The broad anomaly at around 0.8 GPa is clearly revealed by the maximum in the pressure derivative of the frequency of the vibrational modes.
Figure 5. Pressure dependence of the vibrational modes of C70 for selected pressures between 0 and 10 GPa.
detected. (In the MIR region, lower resolution leads to broadening of spectral lines.) The splitting of this line was clear enough to be modeled by fits with Lorentzian lines. We present the parameters obtained by this fit in Figure 3. (Since this frequency is assigned to an A200 mode, the appearance of the second peak is either due to Davydov splitting or to inequivalent environments
of static molecules, as explained in the Introduction.) The frequency (ν*), area (A), and width (w) all show a clear discontinuity around 270 K, the onset of uniaxial rotation of the molecules. The change of rotation from uniaxial to isotropic seems to have a much smaller effect. In the mid-infrared (right panel of Figure 2) three new weak lines appear in phase III in the vicinity of the E10 mode at 728 cm-1. Comparing the frequencies with calculations,20,21 we assign this change to the activation of silent modes. Pressure Dependence. The pressure-dependent MIR absorbance spectra of C70 are shown in Figure 5 for a few selected pressures up to 10 GPa. At the lowest measured pressure, the spectrum does not differ from that at ambient pressure (see Figure 4). The modes at 0.2 GPa can be assigned on the basis of Table 1 to phase II. With increasing pressure, most of the vibrational modes of C70 shift to higher frequencies, except the modes at around 577, 673, 3649
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Figure 8. Infrared absorbance spectra of C70 as a function of pressure for pressures between 0 and 10 GPa, presented with a vertical offset. The numbers indicate the applied pressures in GPa.
Figure 9. Comparison of selected regions of the infrared spectra (from bottom to top) at ambient conditions, at 77 K and at 3 GPa. Curves are vertically offset for clarity.
727, 742, and 795 cm-1. The red shift of the vibrational modes at 577, 673, and 795 cm-1 with increasing pressure is in agreement with the earlier infrared studies under pressure.16,27 Although the intensity of some vibrational modes is reduced drastically at higher pressures, most of the vibrational modes persist up to 10 GPa. Frequencies of some selected vibrational modes as a function of pressure (up to 7 GPa) (obtained by Lorentzian fits) are shown in Figure 6. The frequency of the vibrational modes exhibits a nearly linear pressure dependence above 3.5 GPa with changes occurring below. In most modes, as we reported earlier,28 a clear anomaly is found at about 0.8 GPa, where the linear pressure coefficients (dν/dp) of the vibrational modes change. The anomaly can be illustrated by the pressure derivative (dν/dp) of the frequency of the vibrational modes, plotted in Figure 7 for selected modes, where it appears as a broad maximum.
In addition to the shift of the vibrational modes, their intensities also exhibit changes with increasing pressure. The pressure-dependent spectra of C70 are shown with vertical offset in Figure 8 to illustrate the intensity changes more clearly. Obviously, the weak vibrational modes between 500-570 cm-1, 680-795 cm-1, and 1200-1600 cm-1 increase in intensity with increasing pressure up to 7 GPa. Comparison of Temperature and Pressure Dependence. The changes detailed above at around 270 K and around 0.8 GPa suggest the occurrence of a phase transition. Following the notation based on molecular degrees of freedom, and taking into account that at ambient conditions C70 is in the phase II state, this should be the phase II f phase III transition. The molecular picture of this transition corresponds to the freezing of uniaxial rotation with decreasing temperature or increasing pressure. An analogous behavior was observed for C70 3 C8H8.29,30 3650
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Figure 10. Infrared absorbance spectra in the frequency range 570670 cm-1, where new vibrational features emerge at a pressure of ∼7 GPa. These vibrational modes activated at high pressure remain (with even higher intensity) after pressure release.
Figure 11. Absorption spectra of C70 in the frequency range 30018 000 cm-1 for three pressures between 0 and 10 GPa.
In Figure 9 we compare the low-temperature and highpressure spectra with those at ambient conditions. Apart from slight frequency shifts, the newly appearing lines in the highpressure spectra can all be assigned to the phase III modes summarized in Table 1. The transition in the pressure regime is slow; i.e., it is spread over the pressure range ∼0.5 GPa and thus appears to be sluggish. The reason for the sluggish character of the transition is probably the multitude of structures in phase II mentioned earlier. A similar pressure response was observed in energy dispersive X-ray and thermal conductivity studies.6,10 In the latter, full orientational order (i.e., a complete transformation into phase III) was estimated to appear at 0.7 GPa, the approximate position of the peak in the derivative curves. If we include our data in the “rotational phase diagram” presented by Sundqvist,7 they fall very close to two other points derived from vibrational studies, from infrared27 and Raman measurements,13 respectively, which is somewhat below the estimated boundary line from thermal conductivity.10 It seems that thermal conductivity signals the onset of the orientational transition, whereas the vibrational spectra show significant changes only when the ordering is almost complete, resulting in higher critical pressure values at the same temperature. Polymerization at High Pressure. Up to 6 GPa, the pressureinduced changes in the spectra are fully reversible. Above 7 GPa, however, almost all the vibrational modes begin to broaden and lose intensity drastically making their identification among the superimposed interference fringes more difficult. Furthermore, weak modes appear at the frequencies 605, 625, 633, and 652 cm-1 above 7 GPa, which persist after pressure release (see Figure 10). This behavior indicates chemical bonding between fullerene molecules. C70 polymerization has been much less thoroughly investigated than that of C60 (refs 31-33); the results up to now are summarized in the two review papers by Sundqvist,7,17 including the relevant references. The forming of intermolecular bonds is hindered by the less symmetric shape of the molecule: the favored carbon atoms are those of the “polar pentagons” at each end. Thus, besides the stopping of the rotational motion, molecular orientation is also important in polymerization. This is also the reason why it is even more difficult for the polymerization to progress beyond simple dimerization. These restrictions are not present in C60, so a variety of structures are obtained.
The polymerization “map” of C70 has been only partially determined,17 but there are indications that such reactions can occur at relatively low temperature and pressure. We compare our results to those of Lebedkin et al.,22 who apparently found the best conditions for preferred dimer formation at 1 GPa and 473 K. The new lines appearing in the infrared spectra of their dimers, isolated by chromatography, are in perfect agreement with what we observe at high pressure. The spectra also resemble those of photodimers reported by Rao et al.34 We believe that the “phase boundary” between monomers and dimers decreases to room temperature at 7 GPa (this indeed would fit into the phase diagram given by Sundqvist7), and we detect the signatures of C140 dimers in this case. As pressure causes the C70 molecules line up in a favorable pattern for dimers to form, it is indeed possible that at higher pressure room-temperature dimerization occurs. It is important to note that an amorphization of the sample can be ruled out, since a disappearance of existing modes was not observed in our study. Furthermore, the amorphization of C70 fullerene is expected to occur at higher pressures according to earlier studies.6,35 Pressure Dependence of the Absorption Edge. The infrared absorbance spectra of solid C70 over the whole measured frequency range are shown in Figure 11 for selected pressures between 0 and 10 GPa. (The interference fringes in the spectra are due to multiple reflections of the incident radiation within the diamond anvil cell.) We present the detailed pressure dependence of the absorption edge in Figure 12 for selected pressures in the range 0-10 GPa. The energy Ea of the absorption edge decreases with increasing pressure up to 10 GPa. For a quantitative analysis we consider the absorption edge Ea as the inflection point of the absorption spectrum. We determine this value from the maximum in the first derivative of the absorption spectrum (see inset of Figure 13 for illustration). The energy position was determined by a Gaussian fit. The resulting pressure dependence of Ea is shown in Figure 13 in comparison with the results of Meletov et al.36 C70 is a typical molecular crystal where the absorption edge represents the highest-occupied molecular orbital to lowest unoccupied molecular orbital (HOMO-LUMO) gap.37 Ab initio calculations predict for C60 that the lowest-energy electronic transitions do not occur between extended bands but are rather considered as “on-ball” transitions or local excitons.38 With pressure, two kinds of changes can happen to such modes: a 3651
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transition at 0.8 GPa has no effect on the electronic excitations. Neither did we observe any sign of the dimerization in this region: the absorption edge shift is reversible up to 9.8 GPa. As seen before for polymerized AC60 fullerides,41 polymerization reactions do not influence the visible spectra.
’ CONCLUSIONS We have studied the temperature and pressure dependence of the infrared spectra of C70. We identified distinct phase transitions upon both temperature and pressure which we correlate with the rotational state of the C70 molecules rather than with long-range order in the crystal. This correlation is in accordance with the model of Sundqvist,7,10 who defines the phase diagram as a rotational-orientational rather than a structural representation. At pressures above 7 GPa we observe the formation of dimers at room temperature. The pressure dependence of the absorption edge is in accordance with the formation of Frenkel excitons in these molecular crystals. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (K.K.); christine.kuntscher@physik. uni-augsburg.de (C.A.K.). Present Addresses
)
Figure 12. (a) Transmittance and (b) absorbance spectra of C70 in the NIR-to-visible frequency range for selected pressures between 0 and 10 GPa as well as after pressure release at 1.8 GPa.
Canadian Light Source, Inc., Saskatoon, SK S7N 0X4, Canada. Department of Chemistry, University of Tennessee, Knoxville, TN 37996. ^
Figure 13. Energy position Ea of the absorption edge of C70 as a function of pressure together with earlier experimental results of Meletov et al.36 The inset shows, as an example, the derivative of the absorption spectrum at the lowest pressure with respect to frequency, dA/dω; the energy position of the maximum, as determined from a Gaussian fit (dashed line), served as an estimate for the size of Ea. The data point in green corresponds to Ea on releasing pressure.
continuous red shift due to the broadening of levels, caused by the surrounding molecules, and/or a sudden splitting due to symmetry change at a phase transition. We observe the red shift (Figure 13), which is very similar to previous results by Meletov et al.36 The energy of the absorption edge at ambient pressure, Ea(0), is 1.77 eV. Its pressure dependence is slightly nonlinear (nonlinearity factor of the order of 10-4) with a linear pressure coefficient of about -77 meV/GPa. These values agree reasonably well with previous high-pressure studies15,36,39,40 and are not very different from the predicted pressure dependence of excitonic transitions in C60 (ref 38). As the symmetry change splits the excitonic levels only slightly (presumably less than the broadening on contraction of the crystal), the orientational phase
’ ACKNOWLEDGMENT We thank D. van der Marel and H. S. Somal for their expert help with some of the measurements, performed at the University of Groningen. This work was supported by the German Science Foundation (DFG) and the Hungarian Academy of Sciences under a cooperation grant DFG/183, and the Hungarian National Research Fund under Grant No. OTKA T 75813 and 67866, as well as by the European Commission through the Marie-Curie RTN PRAIRIES (MRTN-CT-2006-035810), and the National Science Foundation (USA) under grant No. DMR0600089. K.K. thanks the Alexander-von-Humboldt Foundation for a Returning Fellowship during which the pressure-dependent work was initiated. ’ REFERENCES (1) Komori, R.; Nagaosa, T.; Hatae, T.; Miyamoto, Y. Jpn. J. Appl. Phys. 1997, 36, 5600. (2) Verheijen, M. A.; Meekes, H.; Meijer, G.; Bennema, P.; de Boer, J. L.; van Smaalen, S.; van Tendeloo, G.; Amelinckx, S.; Muto, S.; van Landuyt, J. Chem. Phys. 1992, 166, 287. (3) Kawamura, H.; Kobayashi, M.; Akahama, Y.; Shinohara, H.; Sato, H.; Saito, Y. Solid State Commun. 1992, 83, 563. (4) Kawamura, H.; Akahama, Y.; Kobayashi, M.; Shinohara, H.; Sato, H.; Saito, Y.; Kikegawa, T.; Shimomura, O.; Aoki, K. J. Phys. Chem. Solids 1993, 54, 1675. (5) Kawamura, H.; Akahama, Y.; Kobayashi, M.; Hasegawa, Y.; Shinohara, H.; Sato, H.; Saito, Y. Jpn. J. Appl. Phys. 1993, 32, L101. (6) Christides, C.; Thomas, I. M.; Dennis, T. J. S.; Prassides, K. Europhys. Lett. 1993, 22, 611. (7) Sundqvist, B. Adv. Phys. 1999, 48, 1. (8) Lundin, A.; Soldatov, A.; Sundqvist, B. Europhys. Lett. 1995, 30, 469. 3652
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dx.doi.org/10.1021/jp200036t |J. Phys. Chem. C 2011, 115, 3646–3653
