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X-ray Absorption Near-Edge Structure (XANES) of Iodine Intercalated C60: Evidence of I2δ+ in I2C60 Nam-Gyu Park, So-Won Cho, and Sung-Jin Kim* Department of Chemistry, Ewha Womans University Seoul 120-750, Korea
Jin-Ho Choy Department of Chemistry, Center for Molecular Catalysis, College of Natural Sciences Seoul National University, Seoul 151-742, Korea Received September 7, 1995 Revised Manuscript Received December 12, 1995 The occurrence of superconductivity upon intercalation of the donor-type alkali metals into the fullerene C60 structure1-4 has stimulated studies on finding a new type of superconductor. The possibility of the superconductivity of iodine-intercalated C60 has been studied; however, no superconductivity down to 4 K was reported.5 The absence of superconducting property is considered to be due to the difference in the electronic properties compared to the alkali-metal-doped system. To understand the electronic structure of the iodine intercalated C60, various spectroscopic measurements employing ESR,6 13C NMR,7 XPS,8 Raman,9 and 129I Mo¨ssbauer10,11 methods together with electrical resistance12 and X-ray diffraction measurements5,13 were carried out, but their characterization has remained controversial because of such contradictory conclusions as the intercalant iodine has acted as electron donor11 or acceptor9 or neither.6-8,10,12,13 To clarify the electronic modifications in iodine upon intercalation into C60, we have performed an X-ray absorption near-edge structure (XANES) spectroscopic analysis for the iodine LI edge before and after intercalation. Since XANES has the peculiar property of being sensitive to electronic change in a selected target atom, it has been used to obtain information about the * To whom all correspondence should be addressed. (1) Hebard, A. F.; Rosseinsky, M. J.; Haddon, R. C.; Murphy, D. W.; Glarum, S. H.; Palatra, T. T. M.; Ramirez, A. P.; Kortan, A. R. Nature 1991, 350, 600. (2) Rosseinsky, M. J.; Ramirez, A. P.; Glarum, S. H.; Murphy, D. W.; Haddon, R. C.; Hebard, A. F.; Palstra, T. T. M.; Kortan, A. R.; Zahurak, S. M.; Makhija, A. V. Phys. Lev. Lett. 1991, 66, 2830. (3) Holczer, K.; Klein, O.; Huang, S.-M.; Kaner, R. B.; Fu, K.-J.; Wetten, R. L.; Diederich, F. Science 1991, 252, 1154. (4) Tanigaki, K.; Ebbesen, T. W.; Saito, S.; Mizuki, J.; Tsai, J. S.; Kubo, Y.; Kuroshima, S. Nature 1991, 352, 222. (5) Zhu,Q.; Cox, D. E.; Fischer, J. E.; Kniaz, K.; McGhie, A. R.; Zhou, O. Nature 1992, 355, 712. (6) Kinoshita, N.; Tanaka, Y.; Tokumoto, M.; Matsumiya, S. J. Phys. Soc. Jpn. 1991, 60, 4032. (7) Maniwa, Y.; Shibata, T.; Mizoguchi, K.; Kume, K.; Kikuchi, K.; Ikemoto, I.; Suzuki, S.; Achiba, Y. J. Phys. Soc. Jpn. 1992, 61, 2212. (8) Werner, H.; Wesemann, M.; Schlogl, R. Europhys. Lett. 1992, 20, 107. (9) Huong, P. V. Solid State Commun. 1993, 88, 23. (10) Grushko, Y. S.; Wortmann, G.; Kovalev, M. F.; Molkanov, L. I.; Ganzha, Y. V.; Ossipyan, Y. A.; Zharikov, O. V. Solid State Commun. 1992, 84, 505. (11) Seto, M.; Maeda, Y.; Matsuyama, T.; Yamaoka, H. Synth. Met. 1993, 55-57, 3167. (12) Akahama, Y.; Kobayashi, M.; Kawamura, H.; Shinohara, H.; Sato, H.; Saito, Y. Solid State Commun. 1992, 82, 605. (13) Kobayashi, M.; Akahama, Y.; Kawamura, H.; Shinohara, H.; Sato, H.; Saito, Y. Solid State Commun. 1992, 81, 93.
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chemical environment of the intercalated species in the intercalated compounds. To discuss the charge-transfer effect, the white line feature of the 2s f 5p transition and the energy shift in the multiple scattering region (or a transition to the underlying continuum state) for the I2C60 XANES were carefully examined. The results were compared with those for the iodine intercalated superconductor IBi2Sr2CaCu2Oy as a reference because the intercalated iodine in this case has been determined to be a negatively charged species as in I3-.14,15 Iodine intercalation into C60 was carried out by vaporphase reaction of iodine with pure C60 at 300 °C for 3 days in an evacuated quartz tube, using starting mixture with the ratio of I:C60 ) 5:1. The excess iodine was removed by transporting to the colder end of the tube after lowering temperature from 300 to about 100 °C. A powder X-ray diffraction pattern showed that the single-phase intercalation compound was obtained, and the diffraction data were analyzed using a Rietveld type full profile refinement technique with a measuring step of 0.02° in 2θ. X-ray absorption experiments were carried out at the beamline 7C of the Photon Factory, National Laboratory for High Energy Physics (KEK-PF). For the measurement of the iodine LI edge, synchrotron radiation from the electron storage ring (2.5 GeV with a stored current of about 300-360 mA) was monochromatized with a silicon (111) double-crystal monochromator which was detuned by 60% to remove high harmonics. All I LIedge XANES spectra for the present samples were recorded in a transmission mode at room temperature. The samples for XANES were prepared in the form of a fine powder uniformly dispersed onto adhesive tape by Nujol and were folded into layers to obtain an optimum edge step (∆µt ≈ 1) and to eliminate pinhole effects as much as possible, both of which may contribute to the so-called thickness effect. All spectra presented here were normalized to the atomic absorption evaluated from the asymptotic trend at high energy after subtracting the background extrapolated from the preedge region. We checked the quality of the iodine intercalated C60 by X-ray diffraction just after the XANES experiment, because there was an experimental report on its instability upon exposure to air.16 Consequently, there was no change in the X-ray diffraction pattern before and after the XANES experiment for the iodineintercalated C60. The stoichiometry of the iodine intercalated C60 was determined to be I2.20(0.03C60 by the Rietveld occupancy refinement, which was consistent with the thermogravimetric analysis.17 From the best fit result obtained from the Rietveld refinement with trigonal P3 h symmetry,18 (14) Faulgues, E.; Russo, R. E. Solid State Commun. 1992, 82, 531. (15) Choy, J.-H.; Kim, D.-K.; Kang, S.-G.; Kim, D.-H.; Hwang, S.J. Superconducting Materials; Etourneau, J., Torrance, J. B., Yamauchi, H., Eds.; IITT-International: Paris, 1993; p 329. (16) Zenner, T.; Zabel, H. J. Phys. Chem. 1993, 97, 8690. (17) In TGA analysis on 30 mg of sample, the weight loss of 29.7% was observed at between 100 and 250 °C, which corresponds to the composition of I2.5(0.5C60. (18) The present diffraction profile could be indexed as trigonal with the lattice parameters of a ) b ) 9.965(1) Å, c ) 9.992(1) Å and γ ) 120°. X-ray pattern has the best fit with the geometry in which iodine atom occupied statistically over the 6g position, instead of 2d and 3f position in P3h space. The R values obtained were Rp ) 3.18% and Rwp ) 4.42%.
© 1996 American Chemical Society
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Figure 2. First-derivative spectra of I LI-edge XANES, where the first peak and the second one represent an edge at 2s f 5p transition and at a transition from 2s to continuum state, respectively. Table 1. Comparison of the Difference in Absorption Edge Energy between LI Edge (EL) and Continuum Edge (EC) for I2, I2C60, and IBi2Sr2CaCu2Oy
Figure 1. Normalized I LI-edge XANES spectra for I2C60 and I2 (top) and those for IBi2Sr2CaCu2Oy and I2 (bottom). The inset in each spectrum shows an expended spectrum at the multiple scattering region.
sample
EL (eV)a
EC (eV)b
∆E ) EC - EL (eV)
I2 I2C60 IBi2Sr2CaCu2Oy
5184.6 5184.5 5184.7
5191.7 5192.3 5190.3
7.1 7.7 5.6
a E corresponds to an edge energy of 2s f 5p resonance L transition, which is determined from the first peak position in the first derivative XANES data. b EC represents an edge energy corresponding to a transition from 2s to continuum state, which is determined from the second peak position in the first derivative XANES data.
the structure of the intercalated iodine was found to be of the molecular type as I2 and its (I-I) bond distance was 2.63 ((0.01) Å, which was somewhat longer than the previously reported value of 2.53 Å.5 However, it is noted that these two values are slightly, but significantly considering the error range, shorter than the intramolecular distance 2.72 Å reported for elemental I2.19 The electron configuration for I2 is σ2π4n4π*4σ*. Therefore, any charge transfer from C60 to an unoccupied iodine σ* level would cause elongation of the (II) bond, whereas bond shortening would be expected upon backward charge transfer. We can therefore consider the (I-I) bond length as an indicator for direction and degree of charge transfer in I2C60. This difference in the (I-I) distance between elemental I2 and molecular iodine in I2C60 is also expected to have an influence on the XANES spectra, which will be discussed hereafter. Figure 1 shows the normalized I LI-edge XANES spectra for I2C60 and IBi2Sr2CaCu2Oy together with that for pure iodine I2 for comparison. The sharp peak at around 5186 eV, called the white line (WL) feature, corresponds to a 2s f 5p transition whose intensity is associated with the unoccupied final 5p states of iodine. Little change in the WL intensity is observed before and after intercalation of iodine into C60, while the WL intensity for IBi2Sr2CaCu2Oy is significantly depressed compared to that for I2 indicating an increase in electron density of the 5p state due to electron transfer from the Bi2Sr2CaCu2Oy lattice to the intercalated iodine. Considering the electronic modification of iodine only from
the WL intensity point of view, it seems that there is no charge transfer between iodine and C60. However, one can also see a difference in the second absorption edge energy between I2C60 and IBi2Sr2CaCu2Oy (see the inset spectra in Figure 1), in which this second edge of the multiple scattering region corresponds to a transition to the underlying continuum state. In the case of I2C60, the second edge energy is shifted to the higher energy side, which is contrary to the direction of the shift for IBi2Sr2CaCu2Oy. These shifts are clear in the first derivative spectra as shown in Figure 2. The edge energies of the 2s f 5p transition (EL) and the 2s f continuum state (EC) are presented in Table 1 along with their difference (∆E ) EC - EL). ∆E is larger for I2C60 and smaller for IBi2Sr2CaCu2Oy than the value for I2. From this, the electronic structure of the intercalated iodine between I2C60 and IBi2Sr2CaCu2Oy is clearly quite different. We can consider that ∆E, associated with the difference in energy between the highest occupied level and the unbound continuum level, is related to the antibonding electronic character. As in the case of the diatomic oxygen or fluorine molecule where the ionization potential, bond dissociation energy and bond distance depend on the antibonding electronic density,20 it is expected that a removal of antibonding electrons in molecular I2 orbital makes ∆E larger and a smaller ∆E will be caused by adding an electron to the antibonding orbital. In this context, we can deduce
(19) van Bolhius, F.; Koster, P. B.; Michelsen, T. J. Acta Crystallogr. 1967, 23, 90.
(20) Pauling, L. The Nature of the Chemical Bond, 3rd ed.; Cornell University Press: Ithaca, NY, 1960.
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the electronic structure of the intercalated iodine from the magnitude of ∆E in the iodine XANES. As mentioned previously, in the case of IBi2Sr2CaCu2Oy, the iodine in the interlayer space is known to be a negatively charged species and thereby the smaller ∆E compared to I2 results from the addition of extra electron donating into the antibonding σ* orbital. For I2C60, the increase in ∆E can be explained by a removal of the electrons in antibonding π* orbital of I2, which leads us to conclude that partial electron transfer occurs from iodine to C60. The removal of antibonding electron density is also expected to make the (I-I) distance shorter than the neutral iodine molecule due to an enhancement of the bonding character. According to the Rietveld refinement result for I2C60, the (I-I) distance is shortened by 0.1 Å compared to neutral I2. With regard to the relationship between the absorption energy and the bond distance, Bianconi has proposed that the energy shift in the multiple scattering region depends on the bond distance, that is E ∝ 1/R2.21 Therefore, a blue shift in the absorption edge energy at the multiple scattering region for the I2C60 spectrum compared to the energy in the I2 spectrum reflects a decrease in the (I-I) bond distance, which is consistent with the present X-ray diffraction analysis. On the basis of all these factors, we propose the formation of I2δ+ in I2C60. Finally, a slight variation of the WL area in the iodine LI-edge XANES spectra before and after intercalation into C60 is also expected based on the above chargetransfer considerations. The WL area is calculated by employing a least-squares fitting program on the XANES spectrum. Each spectrum is fitted with a superimposition of the Lorentzian function and the arctan step function. In the course of calculation, several iterations are performed until the variables converge. The WL area is determined only from integrating the Lorentzian line. Figure 3 shows the bestfit spectra for I2C60 and I2, and the fitting parameters along with the WL area are listed in Table 2. The WL area for I2C60 is very slightly increased compared to that for I2, indicating a slight increase in the density of the unoccupied 5p state. The other evidence to support an electron donation from I2 to C60 has been obtained in various experiments such as ESR and Raman. Our ESR study of I2C60 suggests that the spins are on the C60 fullerene, and the higher shift in ν (I2) frequency (from 212 cm-1 for neutral I2 to 218 cm-1 for the iodine-intercalated C60) observed in our Raman spectra also supports that iodine, at least in the case of I2C60, acts as an electron donor. Moreover, in the susceptibility measurement, the characteristics of ferromagnetism at below ∼20 K have been observed, which is similar to those of the (21) Bianconi, A.; Dell’Ariccia, M.; Durham, P. J.; Pendry, J. B. Phys. Rev. B 1982, 26, 6502.
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Figure 3. Experimental I LI-edge XANES data (closed circles) and the fit to the sum of the Lorentzian and Arctan step functions (heavy solid lines) for I2C60 and I2. The separate components of this function are also shown by the light solid lines. Table 2. Fitting Parameters (H, σ, E0) and Areas (A) of the Lorentzian Line of the I LI-Edge XANES Spectra for I2C60 and I2 sample
Ha
σ (eV)a
E0 (eV)a
Aa
I2C60 I2
1.09 1.11
4.1 3.9
5186.3 5186.3
6.13 5.99
a Symbols H, σ, E , and A represent the maximum amplitude, 0 full width at half-maximum (fwhm), energy at peak maximum, and area of Lorentzian line, respectively, obtained by fitting the following Lorentzian function to the observed XANES data: f(E) ) H[(σ/2)2/((σ/2)2 + (E - E0)2)], where H is represented by k(2/π/ σ) where k is a constant.
electron doped fullerene, C60-TDAE.22 The detailed magnetic behavior will be reported subsequently. Supporting Information Available: Crystal structure data for I2C60 (4 pages). Ordering information is given on any current masthead page.
Acknowledgment. This work was supported by the Korean Ministry of Education (BSRI-95-3413). N.-G. Park acknowledges financial support from the Korean Science and Engineering Foundation (KOSEF) as the postdoctoral fellowship (95-2311-556). CM950412K (22) Allemand, P.-M.; Khemani, K. C.; Koch, A.; Wudl, F.; Holczer, K; Donovan, S.; Gruner, G.; Thompson, J. D. Science 1991, 253, 301.
