1826
J. Phys. Chem. 1995, 99, 1826-1830
Magnetic Properties of Rare-Earth Metallofullerenes Hideyuki Funasaka,* Kenji Sugiyama, Kazunori Yamamoto, and Takeshi Takahashi Nuclear Fuel Technology Development Division, Tokai Works, Power Reactor and Nuclear Fuel Development Corporation, Tokai, Ibaraki, 319-11, Japan Received: October 18, 1994; In Final Form: December 14, 1994@
Bulk amounts of La@Cs;? and Gd@& have been isolated in pure form from various hollow fullerenes. Magnetization data for these powder samples, an isolated La@C82 isomer and a Gd@C82 isomer, have been obtained employing a SQUID magnetometer at temperatures ranging from 3 to 300 K. For La@C82 the inverse susceptibility as a function of temperature follows a Curie-Weiss law. The effective magnetic moment . Gd@C82 the magnetization data fall on a universal curve which is fitted to a per La@Csz is 0.38 p ~ For Brillouin function in correspondence with the Gd3+ free ion ground state values of J = 3.38 and g = 2.
Introduction
Experimental Section
Since techniques to produce metallofullerenes in bulk by laser' or arc vaporization's2 of graphite-metal composites in helium were developed, a number of metallofullerenes which include La,'-'2 Sc,5,8,16317 and most of the lanthanide elementsI8 have been prepared and characterized by many researchers. Metallofullerenes might be expected to have important applications such as superconductors, organic ferromagnets, laser, ferroelectric materials, and so on.I9 In order to open a new stage for these developing applications in metallofullerenes, it is necessary to produce macroscopic quantities of metallofullerenes in pure form and then to explore the properties of these novel materials, Nevertheless, owing to the scarcity of the metallofullerenes of the (M)& type in the soot2 and the difficulty in the separation and isolation of metallofullerenes in pure form,20 species-specific detection techniques such as ESR, XPS, EXAFS, and mass spectrometry have been mainly used for characterizing the metallofullerenes: however, important progress toward purification of metallofullerene La@Ca has recently been reported by two groups using a single-stage HPLC method" and a two-stage method? independently. Here we focus our attention on the magnetic properties of Ln@C82 (Ln = lanthanide) in the solid form. For Ln@C82 magnetism would be associated both with the electronic configuration, especially 4f electrons, and with the electrons transferred to the carbon shell. The electronic configuration for lanthanum is 6s25d'4P, and trivalent lanthanum corresponds to closed shells of electrons, losing their 6s and 5d electrons. Lanthanum metal exhibits weak diamagnetism, and lanthanum carbide nanoparticles also showed a diamagnetic response. Therefore, for La3+@C823-magnetism would be only associated with the electrons transferred to the carbon shell. On the other hand, gadolinium has a 6s25d'4f/ configuration and a large effective magnetic moment, 7.94 p g . The magnetic properties for Gd2C3 nanoparticles have been studied,21and it has been reported that the effective magnetic moment for these is consistent with the Gd3+ free ion ground state value. But the magnetic behavior of bulk purified Ln@Cs2 isomer is still unknown. In this paper, we present the magnetic properties of bulk purified Lam82 and Gd@C82 isomers, measuring them by a SQUID magnetometer.
The samples in this study were produced by employing the same arc discharge m e t h ~ d ' , ~ . ~ -and ' , ~ the ~ same separation as used in the previous studies. Graphite rods (Nippon Carbon Co., 13 mm in diameter, EG-38H, 99.998%) were drilled out (7 mm diameter) and filled with a mixture of La203 (Chori Co., 99.9%) or Gd203 (Soekawa Chemical Co., 99.99%), graphite powder (Soekawa Chemical Co., 99.9%), and prepowder asphalt (Showa Shell Co.,) with a ratio of 5:3:2 by weight. Before use, the rod was cured at 300 "C for 2 h and carbonized at 1000 "C for 1 h in a nitrogen flow. The resulting rod was 1-2 atom % lanthanum or gadolinium. A metalimpregnated graphite rod was used as an anode in a contact arc reactor at 250 A in a He (45 Torr) static atmosphere.'S2 The resultant soot was collected and extracted for 48 h in 1,2,4-trichlorobenzene(TCB, 99.0%) using a Soxhlet extraction apparatus. The extracts were redissolved in toluene prior to high-performance liquid chromatography (HPLC) separation. The toluene solution of the extracts was separated using an HPLC system with a 2-( 1-pyreny1)ethyldimethylsilylated silica (COSMOSIL SPYE, 20 mm id., 250 mm in length, Nacalai Tesque Co.) with toluene eluent (flow rate, 10 mL/ min). In this HPLC separation process (330 nm W detection), the peak due to La@C82or Gd@Cs2was observed between c 8 4 c g 6 and c 8 8 peaks. By collecting the corresponding fraction, we were able to purify and isolate the La@Cs2 and Gd@Cs2 species, respectively. The isolation of La@& and Gd@C82 was then identified by laser desorption Fourier transform ion cyclotron resonance mass spectrometry (LD-FT-ICR) measurements (Extrel FTMS 2001).25 In the mass analysis, the desorption and ionization laser (N2 laser, 337 nm) was maintained at a very low fluence below 100 pJ/1 mm diameter spot on the sample to avoid unnecessary fragmentation. The isomeric purification of La@C82 was confirmed by electron spin resonance (ESR) spectra. Both crude extracts and isolated La@Cg2 were dissolved in TCB and degassed. ESR measurements were performed using a conventional X-band ESR spectrometer (Bruker ESP 300) at room temperature. Furthermore, by repeating collectiodinjection cycles of a corresponding HPLC fraction, we finally obtained the La@Cg2 powder sample of 27.4 mg and Gd@C82powder sample of 12.0 mg Magnetization data for these La@Cs:! and Gd@C82 powder samples have been obtained employing a Quantum Design SQUID magnetometer. Magnetization (H, T) has been deter-
Y,3-6913-15
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Abstract published in Advance ACS Abstracts, February 1, 1995.
0022-3654/95/2099-1826$09.00/0
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Letters
J. Phys. Chem., Vol. 99, No. 7, 1995 1827 distribution of the hollow all-carbon fullerenes such as Cw, c70, C7dC78, CgdCg4, and Cgo resembles that in the extracts of pure carbon soot. As shown in Figure 1, there exists a sharp peak between the Cg4 4-c86 and CSSpeaks. In the pure carbon case, no peaks were observed between the c 8 4 c 8 6 and c88 peaks. This peak corresponds to M@C82 (M = La and Gd). The retention time for Gd@C82 (41 min) is slightly longer than that of La@& (37 min)." By comparison between HPLC spectra of Gd@Cgz and La@Cg2, it is supposed that this single peak in Figure l b corresponds to a major isomer of M@Cg2 (M = La and Gd). Details on the isolation of M@Cg2 will be published elsewhere.26 Parts a and b of Figure 2 reveal the positive ion mass spectra of LD-FT-ICR for this rechromatographed fraction for La and Gd, respectively. The obtained ion intensity ratio approximately agrees with the calculated isotope distribution of M@C82, and these ratios are used as a criterion for the peak assignment. The all-carbon fullerenes other than M@Cg2 and trace amount of M@Cgo fragmented from M@C82 are completely absent in the mass spectra in Figure 2a,b. The X-band ESR spectra of the crude mixture of the lanthanum containing fullerenes and of isolated La@Cg2in TCB at room temperature are shown in Figure 3, a and b, respectively. The two dominant ESR hyperfine patterns for lanthanofullerenes were essentially the same as those reported previou~ly.~.~ On the basis of both FT-ICR mass spectrum and ESR spectrum (Figures 2 and 3), it is concluded that the isolation of the paramagnetic La@Cs;? with the hyperfine coupling constant (HFCC) of 1.15 G and the separation of La@& (octet A with HFCC = 1.15 G) from La@Cgz (octet B with HFCC = 0.83 G) is complete. But it was not possible to measure the ESR spectrum of Gd@Cg2 at room temperature because the spinlattice relaxation time of Gd@& is very much shorter. By
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Figure 1. (a) An overall HPLC profile for the redissolved crude extract from the metallofullerene soot. The M@C82 peak retention time for La@Cs2" is 37 min (dotted line) and for Gd@C82 (full line) is 41 min as seen between peaks of C84 C86 and C S ~ .(b) An isolated HPLC profile for the M@C82 (M = La and Gd). Only one peak is found in (b) except for solvent impurities near 6 min.
+
mined in solenoidal fields between f5.5 T and for temperature ranging from 3 to 200 K.
Results and Discussion Purification. Figure l a shows an HPLC profile of the crude extract of the fullerene soot redissolved in toluene. The
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Figure 2. Laser desorption Fourier transform ion cyclotron resonance mass spectra (a) of the La@Cs2 from 500 to 1500 amu and (b) of the Gd@C82 from 500 to 1500 amu. Positive ions directly were produced by a 337 nm laser irradiation. It is observed that all-carbon fullerenes and metallofullerenes other than M@Csz (M = La and Gd) and a trace amount of M@Cs2 fragmented from M@C82 are completely absent in the mass spectra.
1828 J. Phys. Chem., Vol. 99, No. 7, 1995
Letters approximation of the density functional theory. They found that for lower-symmetry sites a certain off-center metal position is more stable, and the three metal valence electrons transfer to the cage, forming La3+@C8z3-, which is supported by the experimental results. Furthermore, they suggested that some small s-like component of the valence electron density is left on the La ions, in agreement with hyperfine data. Nagase et al.3033’showed similar results by unrestricted Hartree-Fock calculations and pointed out that some small s-like component of the d spin density is left on the La ions, and the density on La becomes as small as 0.084. The magnetic susceptibility of La@C82 was measured from 200 to 5 K under a magnetic field of 5 T (Figure 4a). This is clearly consistent with a paramagnetic state. Figure 4b shows inverse magnetic susceptibility as a function of temperature for fixed applied field 5 T. By fitting to the Curie-Weiss law,
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comparison of an HPLC spectrum of La@C82 with that of Gd@C82, and cyclic and differential pulse voltammograms of La@C82 with those of Gd@C82,*’ it is supposed that isolation of Gd@Cg;! isomer is also complete. Magnetization.
[email protected] shown in Figure 3, the 1.15 G HFCC for the lanthanofullerenes is very small compared with the observed value of 50 G for La2+ substituted in CaF228and the calculated value of 186 G for an isolated La2+ ion. From these results, Johnson et aL2 suggested a picture in which the La 6s electrons pair in the lowest unoccupied molecular orbital of c 8 2 while the La 5d electron is also almost completely transferred to the cage in a rather delocalized state, thus leading to a formal charge on La of close to 3 f . This small isotropic HFCC of La@& also indicates a small electron density of the radical electron in an s orbital of La. Now more advanced electronic structure calculations on La@& have been performed by two groups. Laasonen et al.29investigated an abinitio calculation for Lac82 by means of the local density
(4)
where S and n are the resultant spin and the number of unpaired electrons, respectively. The number of unpaired electrons found from eq 4 is 0.071. This value is approximately consistent with the calculated d-spin density of atomic La. Therefore, it is considered that the transferred electrons to the cage (except for those left on the La ions) are uniformly diffused inside the cage. Gd@& In Figure Sa, magnetization (M) vs field (H) data for this isolated Gd@& powder are shown at temperatures of 3, 5 , 10, 25, 50, 100, 200, and 295 K. Figure 5b shows the same magnetization (M) curves plotted against HIT on the horizontal axis resulting in a universal curve, indicating a paramagnetic response. It is clear that the magnetization of Gd@C82 tends to saturate over 1.5 in HIT. The paramagnetic magnetization A4 as a function of HIT is fit with a Brillouin function B&). = ngJ,BBJ(x)
(5)
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Here n is the number of magnetic atoms per unit volume, g is the Lande g factor, J is the total angular momentum, is the Bohr magneton, and n = gJpBHlkT. The results were fitted well by J = 3.38 with the x2 method32 (so-called two-dimensional curve fitting), taking theoretical value of g = 2.00. This is consistent with a Gd3+ionic ground state 8S7/2 (theoretical value of J = 7/2), as observed with the
Letters
J. Phys. Chem., Vol. 99, No. 7, 1995 1829
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moment calculated from the Curie constant is 6.90 p ~ Both . values are also a little smaller than Gd2C3 nanoparticles peff= 8.28 p~ (J = 3.67 and g = 2)21or the theoretical value of peff = 7.94p ~ For . holmium, it has been reported that the effective magnetic moment of Ho@C82 (the purity of HO@C82 is not mentioned) is smaller than that of H02C3 nan~particles.~~ It is supposed that the reduced moment of Gd@C82 is attributed to the electrons transferred to the carbon shell, taking into account those La@& and Ho@Cg2 results. In conclusion, the effective magnetic moment per La@Ca is 0.38 p~ obtained from the observed inverse susceptibility, which is in approximate correspondence with the calculated spin density on the La atom. For Gd@C82, the magnetization data are consistent with the Gd3+ free ion ground state values of J = 3.38 and g = 2.
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References and Notes (1) Chai, Y.; Guo, T.; Jin, C.; Haufler, R. E.; Chibante, L. P. F.; Fure,
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J.; Wang, L.; Alford, J. M.; Smalley, R. E. J. J . Phys. Chem. 1991, 95,
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Acknowledgment. We express thanks to Mr. Koji Sakurai (Zuiho Sangyo Co.), Toshiaki Ishiguro (Genshiryoku Gijutu Co.), and Yoshiharu Kano (Genshiryoku Gijutu Co.) for their experimental help.
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150
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Figure 6. (a) Magnetization (emdg) vs temperature for isolated Gd@& powder measured in an applied field of 5 T. (b) Inverse susceptibility data plotted as a function of temperature using these data in (a).
gadolinium carbide nanoparticles ( J = 3.67). Furthermore, the magnetic susceptibility of Gd@& was measured from 200 t o 5 K under a magnetic field of 5 T (Figure 6a). Inverse magnetic susceptibility as a function of temperature for fixed applied field of 5 T is shown in Figure 6b. By fitting to the Curie-Weiss law using a least-squares method, a Curie constant C = 5.22 x emu K/(g Oe) (per gram of sample) and Weiss temperature of -0.65 K are yielded. As previously shown, predicting J = 3.38 and g = 2, the effective magnetic moment per Gd3+ is g[J(J 1)]"2pB = 7.70 p ~ and , the effective magnetic
+
7564. (2) Johnson, R. D.; de Vries, M. S . ; Salem, I.; Bethune, D. S.; Yannoni, C. S . Nature 1992, 355, 239. (3) Weaver, J. H.; Chai, Y.; Kroll, G. H.; Jin, C.; Ohno, T. R.; Haufler, R. E.; Guo, T.; Alford, J. M.; Conceicao, J.; Chibante, L. P. F.; Jain, A,; Palmer, G.; Smalley, R. E. Chem. Phys. Lett. 1992, 190, 460. (4) Ross, M. M.; Nelson, H. H.; Callahan, J. H.; MeElvany, S. W. J . Phys. Chem. 1992, 96, 5231. (5) Suzuki, S . ; Kawata, S . ; Shiromm, H.; Yamauchi, K.; Kikuchi, K.; Kato, T.; Achiba, Y. J . Phys. Chem. 1992, 96, 7159. (6) Hoinkis, M.; Yannoni, C. S . ; Bethune, D. S . ; Salem, J. R.; Johnson, R. D.; Crowder, M. S . ; de Vries, M. S. Chem. Phys. Lett. 1992, 198,461. (7) Bandow, S.; Kitagawa, H.; Mitani, T.; Inokuchi, H.; Saito, Y.; Yamaguchi, H.; Hayashi, N.; Sato, H.;Shinohara, H. J . Phys. Chem. 1992, 96, 9609. (8) Kato, T.; Suzuki, S.; Kikuchi, K.; Achiba, Y. J. Phys. Chem. 1993, 97, 13425. (9) Kikuchi, K.; Suzuki, S . ; Nakao, Y.; Nakahara, N.; Wakabayashi, T.; Shinohara, H.; Saito, K.; Ikemoto, I.; Achiba, Y. Chem. Phys. Lett. 1993, 216, 67. (10) Poirier, D. M.; Knupfer, M.; Weaver, J. H.; Andreoni, W.; Laasonen, K.; Parrinello, M.; Bethune, D. S . ; Kikuchi, K.; Achiba, Y. Phys. Rev. B 1994, 49, 17403. (11) Yamamoto, K.; Funasaka, H.; Takahashi, T.; Akasaka, T. J. Phys. Chem. 1994, 98, 2008. (12) Funasaka, H.; Yamamoto, K.; Sakurai, K.; Ishiguro, T.; Sugiyama, K.; Takahashi, T.; Kishimoto, Y. Fullerene Sci. Technol. 1993, 1, 437. (13) Shinohara,H.; Sato, H.; Saito, Y.; Ohkohchi, M.; Ando, Y. J . Phys. Chem. 1992, 96, 3571. (14) McElvany, S. W. J . Phys. Chem. 1992, 96, 4935. (15) Soderholm, L.; Wurz, P.; Lykke, K. R.; Parker, D. H.; Lytle, F. W. J . Phys. Chem. 1992, 96, 7153. (16) Shinohara, H.; Sato, H.; Ohkohchi, M.; Ando, Y.; Kodama, T.; Shida, T.; Kato, T.; Saito, Y. Nature 1992, 357, 52. (17) Yannoni, C. S . ; Hoinkis, M.; de Vries, M. S . ; Bethune, D. S . ; Salem, J. R.; Crowder, M. S.; Johnson, R. D. Science 1992, 256, 1191. (18) Gillan, E.; Yeretzian, C.; Min, K. S . ; Alvarez, M. M.; Whetten, R. L.; Kaner, R. B. J. Phys. Chem. 1992, 96, 6869. (19) Recently, metallofullerene research was reviewed by: Bethune, D. S.; Johnson, R. D.; Salem, J. R.; de Veries, M. S . ; Yannoni, C. S. Narure 1993, 366, 123. (20) Shinohara, H.; Yamaguchi, H.; Hayashi, N.; Sato, H.; Ohkohchi, M.; Ando, Y.; Saito, Y. J. Phys. Chem. 1993, 97, 4259. (21) Majetich, S . A.; Artman, J. 0.; McHenry, M. E.; Nuhfer, N. T.; Staley, S. W. Phys. Rev. B 1993, 48, 16845. (22) Funasaka, H.; Sakurai, K.; Oda, Y.; Yamamoto, K.; Takahashi, T. Submitted to Chem. Phys. Lett. (23) Alvarez, M. M.; Gillan, E. G.; Holczer, K.; Kaner, R. B.; Min, K. S . ; Whetten, R. L. J. Phys. Chem. 1991, 95, 10561. (24) Kimata, K.; Hosoya, K.; Akai, T.; Tanaka, N. J . Org. Chem. 1993. 58, 282.
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(31) Nagase, S.; Kobayashi, K. Chem. Phys. Lett. 1993,214, 57. (32) Awaya, T. Nucl. Instrum. Methods 1983, 212, 311. (33) Diggs, B.; Silva, C.; Brunett, B.; Kirkpatrick, S.; Zhou, A.; Petasis, D.; Nuhfer, N. T.; Majetich, S. A.; McHenry, M. E.; Artman, J. 0.;Staley, S. W. J . Appl. Phys., in press. JP9428303
