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High-resolution 13C NMR studies of the tetragonal two-dimensional polymerized C60 phase
Physica E 8 (2000) 1–4
www.elsevier.nl/locate/physe
High-resolution 13C NMR studies of the tetragonal two-dimensional polymerized C60 phase A. Rezzouka; b , Y. Errammacha , F. Rachdia; ∗ , V. Agafonovc , V.A. Davydovd a Groupe
de Dynamique des Phases CondensÃees, cc 026, UniversitÃe Montpellier II, Pl. E, Bataillon, F-34 095 Montpellier, France b Laboratoire de Physique de Solide, Facultà e des Sciences Dhar El Mehraz, BP 1796, Atlas, Fes, Maroc c Laboratoire de Chimie Physique, Facultà e de Pharmacie, UniversitÃe de Tours, 31 av. Monge, 37200 Tours, France d Institute for High Pressure Physics of the Russian Academy of Sciences, 142092, Troitsk, Moscow Region, Russian Federation Received 7 February 2000; accepted 16 March 2000
Abstract We present NMR data of tetragonal (tet-2D) two-dimensional polymerized C60 obtained under high pressure. By using C MAS NMR, we were able to identify two resonances, a broad and intense resonance around 146 ppm and a small one at 73.5 ppm. The former line has several components due to inequivalent sp2 carbons on the C60 molecules which were distorted by the transformation under high pressure. We attribute the latter line to the sp3 carbons of the interball bondings. The 13 C MAS NMR line-shape simulation of the obtained spectrum is compatible with the suggested polymeric structure where the C60 molecules are connected by [2 + 2] cycloadditions. We also report on 13 C MAS NMR measurements of tet-2D at ambient pressure and at temperature ranging from 298 to 500 K. We veri ed that the FCC structure of the pristine C60 is ◦ recovered by annealing the sample above 200 C. ? 2000 Elsevier Science B.V. All rights reserved.
13
PACS: 61.48.+c; 76.60.−k; 81.40.Vw Keywords: NMR; Polymers; Fullerene
1. Introduction Since the discovery [1] and macroscopic synthesis [2] of the buckminsterfullerene C60 (BF), there have been a number of studies of its behaviour at high pressure [3–10]. They were essentially aimed for testing its properties under extreme conditions. Recently, new ∗ Corresponding author. Tel.: +33-(0)4-67-14-45-10; fax: 33-(0)4-67-14-46-37. E-mail address: [email protected] (F. Rachdi).
phases of solid C60 have been obtained by applying high pressure and temperature to the solid C60 [11– 13]. On the other hand, it is known that C60 can develop cycloaddition reactions of di erent types. It has been reported that exposure to light [14] can cause the polymerization of the molecules into clusters of di erent numbers of molecules. This reaction is reversible, i.e., heating of polymerized lm allows the recovery of pristine C60 . Its natural to think that similar reactions can happen in the C60 solid under pressure at temperatures that prevent the freezing of the C60 molecules
1386-9477/00/$ - see front matter ? 2000 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 9 4 7 7 ( 0 0 ) 0 0 1 4 2 - 9
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A. Rezzouk et al. / Physica E 8 (2000) 1–4
into an orientationally disordered structure. The main characteristic of the C60 polymers [15,16], which include rhombohedral (rh-2D), tetragonal (tet-2D) and orthorhombic (o-1D) phases [11,12], is the shortening of intermolecular bonds, indicative of a strong bonding mechanism. A similar bonding feature has also been reported in other compressed FCC C60 solids [4,17], in alkali-doped fullerides [18–22], in polymerized C60 lms [14], and in experiments of surface deposition of C60 [23]. Recently, Davydov [24] reported that the tet-2D polymerized phase is as stable as the orthorhombic [25] and rhombohedral [26] polymerized phases of C60 . The tet-2D sample was obtained ◦ at 2.2 GPa and 500 C. In the present work we report on 13 C MAS NMR measurements of tet-2D. The observed inequivalent carbons in the studied polymer are discussed in terms of bonding nature between adjacent C60 molecules and distortion from spherical shape of the molecules. 2. Experimental NMR spectra were recorded at a 13 C frequency of 100.4 MHz (9.4 T eld) on ASX400 spectrometer. The spectra were obtained as the Fourier transformation of the free induction decay (FID) after a =2 RF pulse of 8 s pulse length, with repetition times of 200 and 900 s. From 1100 to 3900 scans (NS) were accumulated for each measurement. All 13 C resonance was referenced to tetramethylsilane (TMS). No special care was taken to avoid contamination with oxygen and the sample was kept in air. About 40 mg of C60 was encapsulated in a Pt container and compressed in a belt apparatus for periods not exceeding two hours. The tet-2D sample was obtained at 2.2 GPa ◦ and 500 C. The X-ray di raction patterns of the polymerized fullerene are similar to that published earlier [24]. 3. Results In Fig. 1 we present room-temperature 13 C spectra of the polymerized C60 , tet-2D, phase obtained under high pressure using di erent repetition delays (D1 ) of 200 and 900 s. In both spectra (see Figs. 1(a) and (b)) we observed two isotropic lines, a broad and in-
tense resonance around 146 ppm and a small one at 73.5 ppm, and two sidebands, which appear at 308 and −15 ppm, corresponding to the line at 146 ppm. We attribute the former group of lines to the inequivalent sp2 carbons on the C60 molecules and the line at 73.5 ppm to the sp3 carbons of intermolecular bondings. We also note that the observed sidebands recover a large extent of anisotropy of about 320 ppm, indicating the freezing in of the polymerized C60 molecules (see Fig. 1). No sidebands were detected for the line at 73.5 ppm indicating a very weak chemical shift anisotropy of the corresponding carbons. The former line at 146 ppm exposes a structure that is related to distortion of the C60 molecules induced by transformation under pressure. The main prerequisite of the polymerized phase formation under pressure is the emergence of orientatonal ordered molecular packing that are characterized by close intermolecular contacts of double bonds of adjacent C60 molecules in parallel orientation favourable, along one direction, for the [2 + 2] cycloaddition reaction [14]. Such contact is not present in the two known FCC and SC molecular structures of C60 [28,29]. So the simplest possibility for the compressed planes of tetragonal phase is the one shown in the insets tet-2D of Fig. 1. The presence of the sp3 carbon peak in 13 C MAS NMR spectra of tet-2D (see Fig. 1) con rms the formation of ordered polymerized phase with a possible cycloaddition reaction between the C60 molecules. We turn now to the splitting of the line at 146 ppm, see the 13 C MAS NMR line shape simulation of the observed inequivalent carbons in tet-2D at the line 146 ppm in Fig. 2 and Table 1 for relative intensities. Fig. 3 shows six isotropic components at the following positions 139.5, 141.1, 143.2, 145.6, 147.9 and 149.9 ppm. We suspect the line at 143:2 ± 0:4 ppm to be from unreacted pristine C60 and evaluate its contribution to be about 5% of the total intensity (that could be probably undetected by the X-ray characterization). As expected in such frozen systems of polymerized C60 , we found long spin-lattice relaxation time T1 and very broad static spectra. The disagreement in the relative intensities of sp2 and sp3 carbons calculated from structural studies and obtained from experiment with repetition times of 10 and 200 s comes from the very long T1 of the sp3 carbons, see Table 1. The intensity of sp2 carbons is always overestimated in comparison with the sp3 carbon intensity due to
A. Rezzouk et al. / Physica E 8 (2000) 1–4
3
Fig. 1. 13 C MAS NMR spectra of tet-2D two-dimensional phase with a spinning rate of 10 kHz and a repetition time D1 of (a) 200 s and (b) 900 s. The stars indicate the spinning sidebands of the line at 146 ppm. The inset shows the structural model of this phase. Table 1 Relative intensities of the isotropic lines including the sidebands in tet-2D polymerized phase Position (ppm)
Spectrum at 10 s
Spectrum at 200 s
Spectrum at 900 s
Calculated from Ref. [23]
146-sp2 73.5-sp3
58.6 1.3
54 5.9
52.2 7.9
52 8
Fig. 2. The 13 C NMR line shape simulation of the observed inequivalent carbons in tet-2D at the line around 146 ppm. Note the presence of about 5% of unreacted pristine C60 at 143.2 ppm.
the chemical shift anisotropy relaxation mechanism which is usually more ecient for sp2 carbons. For the tet-2D, the repetition delay D1 of 900 s is long enough to get full relaxation of all spin, compared to D1 of 10 and 200 s, see Table 1. The relaxed structure of tet-2D (see Fig. 1) shows that the fullerene molecules are no longer spheres, but are deformed by the intermolecular bondings, and elongated along the direction of the deformation as reported in Ref. [27]. It is tempting to interpret the ve isotropic lines as due to this distortion from sphericity of the C60 molecule induced by transformation under pressure. On the other hand, our attempt to dope tet-2D polymer by alkali metal in vapour phase, in order to get a charge transfer low-dimensional system was unsuccessful. For this purpose, we have
4
A. Rezzouk et al. / Physica E 8 (2000) 1–4
References
Fig. 3. 13 C MAS NMR spectra of tet-2D with a repetition time D1 of 30 s: (a) before annealing at 298 K, (b) annealing at 400 K, (c) annealing at 500 K. Rotor frequency is varied from 3 kHz to 5 kHz. The stars indicate the spinning sidebands.
annealed our sample at ambient pressure and at temperature ranging from 298 to 500 K as shown in Fig. 3. By annealing the sample above 500 K, we observed a signi cant change of the corresponding 13 C NMR spectrum which shows a single narrow line at 143 ppm suggesting the transformation of the polymer phase to the FCC structure of the pristine C60 which is characterized by the observed resonance (see Fig. 3). Therefore, the tet-2D polymerized C60 seems to dissociate under temperature e ect. 4. Conclusion 13
C MAS NMR measurements allow us to con rm the predicted polymeric structure of the studied C60 phase. From the line shape simulation of the obtained NMR spectrum, the suggested tetragonal 2D polymerized structure where the C60 molecules are connected by [2 + 2] cycloaddition is clearly evidenced. The annealing of the tet-2D polymer phase above 500 leads to the breaking of the intermolecular bondings and the recovery of the pristine C60 (FCC) phase.
[1] H.W. Kroto, J.R. Heath, S.C. O’Brien, R.F. Curl, R.E. Smalley, Nature (London) 318 (1985) 162. [2] W. Krastschmer, L.D. Lamb, K. Fostiropoulos, D.R. Hu man, Nature 347 (1990) 354. [3] M. Nu˜nez-Regueiro, Mod. Phys. Lett. B 6 (1992) 1153–1158. [4] S.J. Duclos, K. Brister, R.C. Haddon, A.R. Kortan, F.A. Thiel, Nature (London) 351 (1991) 380–382. [5] M. Nu˜nez-Regueiro, P. Monceau, A. Rassat, P. Bernier, A. Zahab, Nature (London) 354 (1991) 289–291. [6] J.L. Hodeau, J.M. Tonnerie, B. Bouchet-Favre, M. Nu˜nez-Regueiro, J.J. Capponi, M. Perroux, Phys. Rev. B 50 (1994) 10 311. [7] H. Hirai, K. Kondo, T. Ohwada, Carbon 31 (1993) 1095. [8] F. Moshay et al., Phys. Rev. Lett. 69 (1992) 466. [9] C.S. Yoo et al., Chem. Phys. Lett. 198 (1992) 379. [10] C.S. Yoo, W.J. Nellis, Science (London) 254 (1991) 1489. [11] O. Bethoux, M. Nu˜nez-Regueiro, L. Marques, J.-L. Hodeau, M. Perroux, In Proceedings of the Materials Research Society, Boston, Abstract of Contributed Papers. Materials Research Society, Pittsburgh, Abstract No. G2.9, 1993, p. 202. [12] M. Nu˜nez-Regueiro, L. Marques, J.-L. Hodeau, O. Bethoux, M. Perroux, Phys. Rev. Lett. 74 (1995) 278. [13] Y. Iwasa et al., Science 264 (1994) 1570. [14] A.M. Rao, P. Zhou, K.A. Wang, G.T. Hayer, J.M. Holden, Y. Wang, W.T. Lee, X.-H. Bi, P.C. Eklund, D.S. Cornett, M.A. Duncan, I.J. Amster, Science 259 (1993) 955–957. [15] C. Goze, F. Rachdi, L. Hajji, M. Nu˜nez-Regueiro, L. Marques, J.-L. Hodeau, M. Mehring, Phys. Rev. B 54 (1996) R3676. [16] J.E. Fischer, Science 264 (1994) 1548. [17] K. Aoki et al., J. Phys. Chem. 95 (1991) 9037. [18] O. Chauvet et al., Phys. Rev. Lett. 72 (1994) 2721. [19] S. Pekker et al., Science 265 (1994) 1077. [20] P.W. Stephens et al., Nature (London) 370 (1994) 636. [21] F. Rachdi, C. Goze, L. Hajji, K.F. Their, G. Zimmer, M. Mehring, M. Nu˜nez-Regueiro, Appl. Phys. A 64 (1997) 295. [22] C. Goze, F. Rachdi, M. Apostol, M. Mehring, G. Zimmer, J.E. Fischer, Proceeding of the European Materials Research Society Strasbourg, Synth. Met. 77 (1995) 115. [23] Y. Kuk et al., Phys. Rev. Lett. 70 (1993) 1948. [24] V.A. Davydov, L.S. Kashevarova, A.V. Rakhmanina, A.V. Dzyabchenko, V. Agafonov, H. Allouchi, R. Ceolin, A.V. Dzyabchenko, V.M. Senyavin, H. Szwarc, Phys. Rev. B 58 (1998) 14786. [25] A.M. Rao, P.C. Eklund, J.-L. Hodeau, L. Marques, M. Nu˜nez-Regueiro, Phys. Rev. B 57 (1997) 4766. [26] C.H. Xu, G.E. Scuseria, Phys. Rev. B 74 (1995) 274. [27] M. Nu˜nez-Regueiro, L. Marques, J.-L. Hodeau, O. Bethoux, M. Perroux, Phys. Rev. Lett. 74 (1995) 278. [28] W.I.F. David, R.M. Ibberson, T.J.S. Dennis, J.P. Hare, K. Prassides, Europhys. Lett. 18 (1992) 219. [29] A. Lundin, B. Sundqvist, Phys. Rev. B 53 (1996) 8329.
www.elsevier.nl/locate/physe
High-resolution 13C NMR studies of the tetragonal two-dimensional polymerized C60 phase A. Rezzouka; b , Y. Errammacha , F. Rachdia; ∗ , V. Agafonovc , V.A. Davydovd a Groupe
de Dynamique des Phases CondensÃees, cc 026, UniversitÃe Montpellier II, Pl. E, Bataillon, F-34 095 Montpellier, France b Laboratoire de Physique de Solide, Facultà e des Sciences Dhar El Mehraz, BP 1796, Atlas, Fes, Maroc c Laboratoire de Chimie Physique, Facultà e de Pharmacie, UniversitÃe de Tours, 31 av. Monge, 37200 Tours, France d Institute for High Pressure Physics of the Russian Academy of Sciences, 142092, Troitsk, Moscow Region, Russian Federation Received 7 February 2000; accepted 16 March 2000
Abstract We present NMR data of tetragonal (tet-2D) two-dimensional polymerized C60 obtained under high pressure. By using C MAS NMR, we were able to identify two resonances, a broad and intense resonance around 146 ppm and a small one at 73.5 ppm. The former line has several components due to inequivalent sp2 carbons on the C60 molecules which were distorted by the transformation under high pressure. We attribute the latter line to the sp3 carbons of the interball bondings. The 13 C MAS NMR line-shape simulation of the obtained spectrum is compatible with the suggested polymeric structure where the C60 molecules are connected by [2 + 2] cycloadditions. We also report on 13 C MAS NMR measurements of tet-2D at ambient pressure and at temperature ranging from 298 to 500 K. We veri ed that the FCC structure of the pristine C60 is ◦ recovered by annealing the sample above 200 C. ? 2000 Elsevier Science B.V. All rights reserved.
13
PACS: 61.48.+c; 76.60.−k; 81.40.Vw Keywords: NMR; Polymers; Fullerene
1. Introduction Since the discovery [1] and macroscopic synthesis [2] of the buckminsterfullerene C60 (BF), there have been a number of studies of its behaviour at high pressure [3–10]. They were essentially aimed for testing its properties under extreme conditions. Recently, new ∗ Corresponding author. Tel.: +33-(0)4-67-14-45-10; fax: 33-(0)4-67-14-46-37. E-mail address: [email protected] (F. Rachdi).
phases of solid C60 have been obtained by applying high pressure and temperature to the solid C60 [11– 13]. On the other hand, it is known that C60 can develop cycloaddition reactions of di erent types. It has been reported that exposure to light [14] can cause the polymerization of the molecules into clusters of di erent numbers of molecules. This reaction is reversible, i.e., heating of polymerized lm allows the recovery of pristine C60 . Its natural to think that similar reactions can happen in the C60 solid under pressure at temperatures that prevent the freezing of the C60 molecules
1386-9477/00/$ - see front matter ? 2000 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 9 4 7 7 ( 0 0 ) 0 0 1 4 2 - 9
2
A. Rezzouk et al. / Physica E 8 (2000) 1–4
into an orientationally disordered structure. The main characteristic of the C60 polymers [15,16], which include rhombohedral (rh-2D), tetragonal (tet-2D) and orthorhombic (o-1D) phases [11,12], is the shortening of intermolecular bonds, indicative of a strong bonding mechanism. A similar bonding feature has also been reported in other compressed FCC C60 solids [4,17], in alkali-doped fullerides [18–22], in polymerized C60 lms [14], and in experiments of surface deposition of C60 [23]. Recently, Davydov [24] reported that the tet-2D polymerized phase is as stable as the orthorhombic [25] and rhombohedral [26] polymerized phases of C60 . The tet-2D sample was obtained ◦ at 2.2 GPa and 500 C. In the present work we report on 13 C MAS NMR measurements of tet-2D. The observed inequivalent carbons in the studied polymer are discussed in terms of bonding nature between adjacent C60 molecules and distortion from spherical shape of the molecules. 2. Experimental NMR spectra were recorded at a 13 C frequency of 100.4 MHz (9.4 T eld) on ASX400 spectrometer. The spectra were obtained as the Fourier transformation of the free induction decay (FID) after a =2 RF pulse of 8 s pulse length, with repetition times of 200 and 900 s. From 1100 to 3900 scans (NS) were accumulated for each measurement. All 13 C resonance was referenced to tetramethylsilane (TMS). No special care was taken to avoid contamination with oxygen and the sample was kept in air. About 40 mg of C60 was encapsulated in a Pt container and compressed in a belt apparatus for periods not exceeding two hours. The tet-2D sample was obtained at 2.2 GPa ◦ and 500 C. The X-ray di raction patterns of the polymerized fullerene are similar to that published earlier [24]. 3. Results In Fig. 1 we present room-temperature 13 C spectra of the polymerized C60 , tet-2D, phase obtained under high pressure using di erent repetition delays (D1 ) of 200 and 900 s. In both spectra (see Figs. 1(a) and (b)) we observed two isotropic lines, a broad and in-
tense resonance around 146 ppm and a small one at 73.5 ppm, and two sidebands, which appear at 308 and −15 ppm, corresponding to the line at 146 ppm. We attribute the former group of lines to the inequivalent sp2 carbons on the C60 molecules and the line at 73.5 ppm to the sp3 carbons of intermolecular bondings. We also note that the observed sidebands recover a large extent of anisotropy of about 320 ppm, indicating the freezing in of the polymerized C60 molecules (see Fig. 1). No sidebands were detected for the line at 73.5 ppm indicating a very weak chemical shift anisotropy of the corresponding carbons. The former line at 146 ppm exposes a structure that is related to distortion of the C60 molecules induced by transformation under pressure. The main prerequisite of the polymerized phase formation under pressure is the emergence of orientatonal ordered molecular packing that are characterized by close intermolecular contacts of double bonds of adjacent C60 molecules in parallel orientation favourable, along one direction, for the [2 + 2] cycloaddition reaction [14]. Such contact is not present in the two known FCC and SC molecular structures of C60 [28,29]. So the simplest possibility for the compressed planes of tetragonal phase is the one shown in the insets tet-2D of Fig. 1. The presence of the sp3 carbon peak in 13 C MAS NMR spectra of tet-2D (see Fig. 1) con rms the formation of ordered polymerized phase with a possible cycloaddition reaction between the C60 molecules. We turn now to the splitting of the line at 146 ppm, see the 13 C MAS NMR line shape simulation of the observed inequivalent carbons in tet-2D at the line 146 ppm in Fig. 2 and Table 1 for relative intensities. Fig. 3 shows six isotropic components at the following positions 139.5, 141.1, 143.2, 145.6, 147.9 and 149.9 ppm. We suspect the line at 143:2 ± 0:4 ppm to be from unreacted pristine C60 and evaluate its contribution to be about 5% of the total intensity (that could be probably undetected by the X-ray characterization). As expected in such frozen systems of polymerized C60 , we found long spin-lattice relaxation time T1 and very broad static spectra. The disagreement in the relative intensities of sp2 and sp3 carbons calculated from structural studies and obtained from experiment with repetition times of 10 and 200 s comes from the very long T1 of the sp3 carbons, see Table 1. The intensity of sp2 carbons is always overestimated in comparison with the sp3 carbon intensity due to
A. Rezzouk et al. / Physica E 8 (2000) 1–4
3
Fig. 1. 13 C MAS NMR spectra of tet-2D two-dimensional phase with a spinning rate of 10 kHz and a repetition time D1 of (a) 200 s and (b) 900 s. The stars indicate the spinning sidebands of the line at 146 ppm. The inset shows the structural model of this phase. Table 1 Relative intensities of the isotropic lines including the sidebands in tet-2D polymerized phase Position (ppm)
Spectrum at 10 s
Spectrum at 200 s
Spectrum at 900 s
Calculated from Ref. [23]
146-sp2 73.5-sp3
58.6 1.3
54 5.9
52.2 7.9
52 8
Fig. 2. The 13 C NMR line shape simulation of the observed inequivalent carbons in tet-2D at the line around 146 ppm. Note the presence of about 5% of unreacted pristine C60 at 143.2 ppm.
the chemical shift anisotropy relaxation mechanism which is usually more ecient for sp2 carbons. For the tet-2D, the repetition delay D1 of 900 s is long enough to get full relaxation of all spin, compared to D1 of 10 and 200 s, see Table 1. The relaxed structure of tet-2D (see Fig. 1) shows that the fullerene molecules are no longer spheres, but are deformed by the intermolecular bondings, and elongated along the direction of the deformation as reported in Ref. [27]. It is tempting to interpret the ve isotropic lines as due to this distortion from sphericity of the C60 molecule induced by transformation under pressure. On the other hand, our attempt to dope tet-2D polymer by alkali metal in vapour phase, in order to get a charge transfer low-dimensional system was unsuccessful. For this purpose, we have
4
A. Rezzouk et al. / Physica E 8 (2000) 1–4
References
Fig. 3. 13 C MAS NMR spectra of tet-2D with a repetition time D1 of 30 s: (a) before annealing at 298 K, (b) annealing at 400 K, (c) annealing at 500 K. Rotor frequency is varied from 3 kHz to 5 kHz. The stars indicate the spinning sidebands.
annealed our sample at ambient pressure and at temperature ranging from 298 to 500 K as shown in Fig. 3. By annealing the sample above 500 K, we observed a signi cant change of the corresponding 13 C NMR spectrum which shows a single narrow line at 143 ppm suggesting the transformation of the polymer phase to the FCC structure of the pristine C60 which is characterized by the observed resonance (see Fig. 3). Therefore, the tet-2D polymerized C60 seems to dissociate under temperature e ect. 4. Conclusion 13
C MAS NMR measurements allow us to con rm the predicted polymeric structure of the studied C60 phase. From the line shape simulation of the obtained NMR spectrum, the suggested tetragonal 2D polymerized structure where the C60 molecules are connected by [2 + 2] cycloaddition is clearly evidenced. The annealing of the tet-2D polymer phase above 500 leads to the breaking of the intermolecular bondings and the recovery of the pristine C60 (FCC) phase.
[1] H.W. Kroto, J.R. Heath, S.C. O’Brien, R.F. Curl, R.E. Smalley, Nature (London) 318 (1985) 162. [2] W. Krastschmer, L.D. Lamb, K. Fostiropoulos, D.R. Hu man, Nature 347 (1990) 354. [3] M. Nu˜nez-Regueiro, Mod. Phys. Lett. B 6 (1992) 1153–1158. [4] S.J. Duclos, K. Brister, R.C. Haddon, A.R. Kortan, F.A. Thiel, Nature (London) 351 (1991) 380–382. [5] M. Nu˜nez-Regueiro, P. Monceau, A. Rassat, P. Bernier, A. Zahab, Nature (London) 354 (1991) 289–291. [6] J.L. Hodeau, J.M. Tonnerie, B. Bouchet-Favre, M. Nu˜nez-Regueiro, J.J. Capponi, M. Perroux, Phys. Rev. B 50 (1994) 10 311. [7] H. Hirai, K. Kondo, T. Ohwada, Carbon 31 (1993) 1095. [8] F. Moshay et al., Phys. Rev. Lett. 69 (1992) 466. [9] C.S. Yoo et al., Chem. Phys. Lett. 198 (1992) 379. [10] C.S. Yoo, W.J. Nellis, Science (London) 254 (1991) 1489. [11] O. Bethoux, M. Nu˜nez-Regueiro, L. Marques, J.-L. Hodeau, M. Perroux, In Proceedings of the Materials Research Society, Boston, Abstract of Contributed Papers. Materials Research Society, Pittsburgh, Abstract No. G2.9, 1993, p. 202. [12] M. Nu˜nez-Regueiro, L. Marques, J.-L. Hodeau, O. Bethoux, M. Perroux, Phys. Rev. Lett. 74 (1995) 278. [13] Y. Iwasa et al., Science 264 (1994) 1570. [14] A.M. Rao, P. Zhou, K.A. Wang, G.T. Hayer, J.M. Holden, Y. Wang, W.T. Lee, X.-H. Bi, P.C. Eklund, D.S. Cornett, M.A. Duncan, I.J. Amster, Science 259 (1993) 955–957. [15] C. Goze, F. Rachdi, L. Hajji, M. Nu˜nez-Regueiro, L. Marques, J.-L. Hodeau, M. Mehring, Phys. Rev. B 54 (1996) R3676. [16] J.E. Fischer, Science 264 (1994) 1548. [17] K. Aoki et al., J. Phys. Chem. 95 (1991) 9037. [18] O. Chauvet et al., Phys. Rev. Lett. 72 (1994) 2721. [19] S. Pekker et al., Science 265 (1994) 1077. [20] P.W. Stephens et al., Nature (London) 370 (1994) 636. [21] F. Rachdi, C. Goze, L. Hajji, K.F. Their, G. Zimmer, M. Mehring, M. Nu˜nez-Regueiro, Appl. Phys. A 64 (1997) 295. [22] C. Goze, F. Rachdi, M. Apostol, M. Mehring, G. Zimmer, J.E. Fischer, Proceeding of the European Materials Research Society Strasbourg, Synth. Met. 77 (1995) 115. [23] Y. Kuk et al., Phys. Rev. Lett. 70 (1993) 1948. [24] V.A. Davydov, L.S. Kashevarova, A.V. Rakhmanina, A.V. Dzyabchenko, V. Agafonov, H. Allouchi, R. Ceolin, A.V. Dzyabchenko, V.M. Senyavin, H. Szwarc, Phys. Rev. B 58 (1998) 14786. [25] A.M. Rao, P.C. Eklund, J.-L. Hodeau, L. Marques, M. Nu˜nez-Regueiro, Phys. Rev. B 57 (1997) 4766. [26] C.H. Xu, G.E. Scuseria, Phys. Rev. B 74 (1995) 274. [27] M. Nu˜nez-Regueiro, L. Marques, J.-L. Hodeau, O. Bethoux, M. Perroux, Phys. Rev. Lett. 74 (1995) 278. [28] W.I.F. David, R.M. Ibberson, T.J.S. Dennis, J.P. Hare, K. Prassides, Europhys. Lett. 18 (1992) 219. [29] A. Lundin, B. Sundqvist, Phys. Rev. B 53 (1996) 8329.