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Heterometallic fullerides of Fe and Cu groups with the composition K2MC60 (M=Fe+2, Fe+3, Co+2, Ni+2, Cu+1, Cu+2, Ag+1)
Journal of Physics and Chemistry of Solids 65 (2004) 337–342 www.elsevier.com/locate/jpcs
Heterometallic fullerides of Fe and Cu groups with the composition K2MC60 (M ¼ Feþ2, Feþ3, Coþ2, Niþ2, Cuþ1, Cuþ2, Agþ1) B.M. Bulycheva, R.A. Luninb, A.V. Krechetovb, V.A. Kulbachinskiib,*, V.G. Kytinb, K.V. Poholoka, K. Lipsc, J. Rappichc a
b
Department of Chemistry, University of Moscow State, GSP-2, Moscow 119992, Russia Department of Low Temperature Physics, Faculty of Physics, Moscow State University, GSP-2, Moscow 119992, Russia c Abteilung Silizium-Photovoltaik, Hahn-Meitner-Institute, D-12489 Berlin, Germany Accepted 15 October 2003
Abstract Heterometallic fullerides with composition K2MC60, synthesized by exchange chemical reaction of K5C60 or K4C60 with chlorides of metals Fe and Cu groups have been investigated by X-ray diffraction, magnetic resonance, Raman and Mo¨ssbauer spectroscopy. Magnetization and susceptibility measurements have also been carried out. Metal chlorides from Fe and Cu groups enable to cover the whole range of electronic configuration of metal from d5 to d10. Heterometallic fullerides with M ¼ Cuþ2, Feþ2, Feþ3 and Niþ2 appeared to be superconductors with Tc ¼ 13:9 – 16:5 K. Ferromagnetism and superconductivity coexist in investigated fulleride K2Feþ3C60. q 2003 Elsevier Ltd. All rights reserved. Keywords: A. Fullerides; A. Magnetic materials; B. Chemical synthesis; D. Superconductivity
1. Introduction Since the discovery of the fullerene C60 [1], a lot of its compounds revealed a wide variety of interesting physical properties. The most fascinating of them was the discovery of superconductivity in alkali doped fullerite [2]. The superconducting properties of fullerides detected for homo and heteronuclear compounds of alkali metals [2 –6] were discovered subsequently, for alkali-earth and some rearearth metals [7 – 14]. The intercalation of multivalent metallic atoms in fullerite lattice voids causes the consecutive filling by electrons of t1u sublevel and higherlying t1g sublevel of a fullerene molecule. Trending efforts on synthesis of heterometallic fullerides with composition K2MC60, we chose metal chlorides from Fe and Cu groups as an objects of further investigations. These chlorides allow covering the whole range of electronic configuration of metal from d5 to d10. Here we present measurement of X-ray diffraction, magnetic resonance, Raman and Mo¨ssbauer spectroscopy in the new heterometallic fullerides. In order to determine * Corresponding author. Tel.: þ 7-95-939-1147; fax: þ7-95-932-8876. E-mail address: [email protected] (V.A. Kulbachinskii). 0022-3697/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2003.10.064
critical temperature of the superconducting transition the ac susceptibility measurements were carried out. The new method of synthesis was developed, based on homogeneous –heterogeneous oxidation –reduction reactions of potassium fullerides with Feþ2, Feþ3, Coþ2, Niþ2, Cuþ1, Cuþ2 and Agþ1 chlorides in organic solvents at temperature T , 340 K. For the samples with the assumed composition K2FeC60 our Mo¨ssbauer spectroscopy studies indicate the presence of Fe atoms in the crystal lattice of heterometallic fulleride. In addition, for these samples the electron spin resonance (ESR) data are presented.
2. Experimental The potassium fulleride was prepared by reaction of fullerite with the calculated quantity of metal in the environment of toluene at 120 – 130 8C [15]. The synthesis of initial alkali metal fullerides, the elimination of toluene, the mixing of fulleride with a suspension of metal halogenide and the realization of an exchange reaction, the drying and prepacking of heterometallic fullerides in ampoules for different physical and spectroscopic investigations were carried out at vacuum in all-soldered glass
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facility. As an example, K2Feþ3C60, KFeþ2 2 C 60 and K2Niþ2C60 were synthesized using following chemical reactions K5 C60 þ Feþ3 Cl3 ! K2 Feþ3 C60 þ 3KCl K5 C60 þ 2Feþ2 Cl2 ! KFeþ2 2 C60 þ 4KCl K4 C60 þ Niþ2 Cl2 ! K2 Niþ2 C60 þ 2KCl The mixture of potassium fulleride and metal chloride is placed for 15 day in the oven with temperature 65 – 75 8C and periodically intermixed by shaking. After the end of reaction, the solvent is removed from the reactor and the precipitate is dried in vacuum. For different measurements, the dry powdered agent was placed in ampoule, which was soldered after filling by helium at pressure 350 –400 mm of mercury. The reactions were controlled by X-ray method. The X-ray data of fullerides had been recorded by photomethod with Ginie chamber (radiation lCu Ka) and listed in Table 1. As the reference, the data for K3C60 synthesized by method [15] is also shown. According to the X-ray data in all obtained precipitations the phases of metals, free fullerite and initial substances are absent. As the main phases the one of two phases of potassium halogenides (KCl or KI) are registered. This fact indicates an execution of an oxidation –reduction reaction. As it is seen in Table 1 parameter a of fcc lattice of heterofullerides less than the value of a in K3C60, synthesized under the same conditions. It seems to be reasonable because the dimensions of ions for metals of Fe and Cu groups are less, hence metals were intercalated to fulleride lattice. Laser induced ion mass spectrometry analysis was made by LAMMA-1000 device (Germany, Leybold AG) using Qswitched Nd-YAG laser (l ¼ 266 nm, t ¼ 8 2 10 ns) with power density on the sample surface 108 – 1011 W/cm2. According to these data, the structure of C60 molecule in heterofulleride is the same as in the host potassium fulleride. In the present work, we used the ac susceptibility measurements to determine the critical temperature of the superconducting transition. The samples were soldered inside the glass ampoules, thus the low frequency inductive method of measurements appeared to be very
suitable. This method is based on the Meissner– Ochsenfeld effect. The measurements were carried out at the frequency f ¼ 22:7 Hz. The magnitude of the applied magnetic field B ¼ 0:5 mT. This signal is detected by ‘Lock in’ amplifier and for control is displayed on the oscilloscope. The magnetic susceptibility x was measured in the temperature range 4:2 , T , 28 K. Magnetic resonance was studied in X-Band (9 GHz) Bruker Elexsys 580 ESR Spectrometer. A gas-flow Oxford Cryostat was used to control the temperature in the range 5 – 300 K. Raman spectra were measured by Perkin Elmer Raman Spectrometer at room temperature. For excitation 632.7 nm red line of He – Ne laser was used. Each spectrum was accumulated during 300 s. Magnetization curves were recorded by vibrating sample magnetometer EG&G PRINCETON APPLIED RESEARCH (USA), model 155, equipped by gas-flow cryostat.
3. Results and discussion 3.1. Ac-susceptibility The temperature of superconducting transition is determined as the onset of the transition. Fig. 1 shows the dependence of the magnetic susceptibility on temperature for fullerides with the composition K2MC60 (M ¼ Fe, Ni, Cu). As the reference sample K3C60 also is shown. Critical temperatures TC are listed in Table 1. Heterometallic fullerides with composition K2MC60 (M ¼ Agþ1, Cuþ1, Coþ2) and KMþ2 2 C60 (M ¼ Co, Ni, Fe) are not superconductors. The magnetic susceptibility data of K2CoC60 and KCo2C60 shows that these samples are paramagnetics down to 4.2 K. Magnetic susceptibility measurements showed that exchange reaction of K3C60 with Cuþ1 Cl and Agþ1 Cl did not produce superconducting heterometallic fullerides. This reaction reduces metal down to zero-valence state (Table 1)
Table 1 Assumed composition Tc (K) Electron configuration Lattice parameter ˚) of fullerides of metal ions a (A K3C60 K2Agþ1C60 K2Cuþ1C60 K2Cuþ2C60 K2Feþ2C60 K2Feþ3C60 KFeþ2 2 C60 K2CoC60 KCo2C60 K2NiC60 KNi2C60
18.5 – – 14.5 16.5 15 – – – 13.9 –
4d10 3d10 3d9 3d5 3d6 3d6 3d7 3d7 3d8 3d8
14.3110(5) – – – 14.24(6) 14.23(2) 14.28(2) 14.264(4) – 14.244(3) –
Fig. 1. Temperature dependence of the magnetic susceptibility of samples with the composition K2MC60 (M ¼ Fe, Ni, Cu). K3C60 is shown as the reference.
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and produces its own phase. For example, in the reaction of K4C60 with CuCl2 the fulleride K2Cuþ2C60 was synthesized. Despite the small size of diameter d of the copper ion Cuþ2 ˚ ), K2Cuþ2C60 is the superconductor with the (d ¼ 0:69 A critical temperature Tc ¼ 14:5 K. Thus, not filled d-shell in intercalated metal leads to superconductivity of fullerides in contrast to f-elements, for which superconductivity was observed in fullerides with filled f-shell. Indeed in the ˚ ), reactions of potassium fullerides with Feþ3 Cl (d ¼ 0:64 A þ2 þ2 ˚ ˚ Fe Cl (d ¼ 0:76 A), and Ni Cl (d ¼ 0:72 A) superconducting materials with composition K2MC60 were synthesized with Tc ¼ 15 2 16 K. 3.2. Mo¨ssbauer spectroscopy Mo¨ssbauer spectroscopy is a powerful tool in the determination of the valence of Fe and its positioning in the lattice [16] The Fe nuclei can experience a magnetic hyperfine field leading to a sextet in the observed Mo¨ssbauer spectrum (Fig. 2a). This spectrum is observed in to the fulleride with the expected composition KFeþ2 2 C60. The measurements were carried out at room temperature T ¼ 298 K. The parameters of sextet are isomer shift IS ¼ 0.01(0) mm/s, internal effective magnetic field Heff ð0Þ ¼ 323:9ð1Þ kOe, and an effective quadrupole moment which is equal to zero. It means that most iron in this sample is a pure a-Fe. However, a feature indicated in Fig. 2a shows the presence of some iron intercalated into fulleride lattice. Mo¨sbauer spectrum is very similar for both K2Feþ2C60 (Fig. 2b) and K2Feþ3C60 fullerides (Fig. 2b). The presence of two significant peaks (marked in Fig. 2b by two solid lines), and the width of the signal G ¼ 1:5 mm/s means that the spectrum consists of two different signals. The first signal has IS ¼ 0.09 mm/s and G ¼ 0:7 mm/s, and the second’s IS ¼ 0.5 mm/s and G ¼ 0:7 mm/s. Large width of each line means that there is quadrupole splitting of the signals ( D ¼ 0:71 mm/s for the first signal and D ¼ 0:86 mm/s for the second). The resulting fitting lines are plotted in Fig. 2b by two solid Lorentzian curves [16]. The isomer shift of the first signal is typical for Fe0 and IS of the second one is very close to Feþ3. This result is applicable also for the fulleride with the assumed
Fig. 2. Mossbauer spectra: (a) a-Fe and (b) K2Feþ2C60. Solid lines are Lorentzian fitting to the experimental data. Two short vertical lines indicate positions of two peaks of different signals.
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composition K2Feþ3C60. So, we suppose that iron intercalates potassium fulleride with valences 0 and þ 3. Quadrupole splitting can be attributed to the presence of Fe in fullerides lattice at minimum of two inequivalent positions. 3.3. Magnetic resonance The typical magnetic resonance spectra at 15 K for K3C60 is shown in Fig. 3. The spectrum is asymmetric and can be fitted with two Lorentzian lines. The width of lines increases with the increase of temperature, while the amplitude of the lines decreases. The g-values of both lines are almost temperature independent. The large fluctuations of the background signal were observed in resonance curve at temperatures below 19 K. The fluctuations vanished at temperatures higher 19 K. Magnetic resonance in K3C60 is ESR [17]. ESR signal is 32 32 due to conducting electrons, C32 60 , C60 –O– C60 and other 32 anions [18,19]. The localized anions (mainly C32 60 – O – C60 in K3C60) give narrow line with g-value about nearly two [19]. It is reasonable to associate broad line observed in our measurements with the signal from conducting electrons, although a small contribution from localized C32 60 anions cannot be excluded. The narrow line can be associated with 32 localized C32 60 – O – C60 anions which are weakly connected with crystalline lattice. The double integrated ESR intensity is proportional to the paramagnetic magnetization or paramagnetic susceptibility of the sample [17]. In K3C60 conducting electrons obey Fermi statistics, while localized C60 anions obey Boltzmann statistics. Therefore, contribution to the magnetization from conducting electrons is almost temperature independent, while the contribution from C60-anions follows Curie –Weis law. The double integrated intensity of magnetic resonance is plotted in Fig. 4 as a function of temperature. Solid line represents the fit of the dependence by the sum of Curie – Weis law and constant. The model
Fig. 3. ESR spectrum of K3C60 recorded at 15 K. Points are experimental data. Solid line is the best fitting of the experimental spectrum by two Lorentzian lines.
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Fig. 4. Double integrated intensity of ESR signal from K3C60 sample as a function of temperature.
function describes an experimental dependence with a good agreement. This gives additional evidence that ESR signal comes from conducting electrons and localized anions. Typical spectrum of magnetic resonance for K2FeC60 is shown in Fig. 5a for a sample with Feþ3. For reference, the resonance curve for K3C60 is shown in Fig. 5b. The resonance curves have almost ideal Lorentzian shape and are essentially broader in Fe-containing compound than in K3C60 (Fig. 5). The amplitude of curve decreases with the increase of temperature, while the width of the resonance increases (Fig. 6). The position of the resonance is sensitive to the sweep range and sweep direction of magnetic field. The fluctuations of the background signal were observed in K2FeC60 as in the case of K3C60. These fluctuations
Fig. 5. ESR spectrum of (a) K2Feþ3C60 recorded at 20 K (points are experimental data, solid line is the best fitting with Lorentzian line) and (b) K3C60 recorded at 20 K.
Fig. 6. ESR spectra of (a) K2Feþ2C60 and (b) K2Feþ3C60 recorded at different temperatures.
disappear at temperatures above the temperature of superconducting transition determined from magnetization measurements. The width of magnetic resonance measured on K2FeC60 fullerides is significantly larger, than the width of ESR curves measured on K3C60, while the amplitude of the signal is much smaller. All resonance curves measured on heterofullerides containing Fe can be very well fitted by one Lorentzian line. If the observed resonance is ESR, one should assume the same time of spin relaxation and same gfactor values for all spins contributing to the observed signal 32 32 (conducting electrons, localized C32 60 and C60 – O – C60 — anions and Fe ions). This means that the intercalation of Fe causes the strong interaction between magnetic moments in fulleride. More reasonable interpretation is that the spins in fulleride are ordered and observed magnetic resonance is ferromagnetic resonance. The g-factor for the ferromagnetic resonance is often close to two [20], but sensitive to the shape of sample. Calculated g-factor values obtained for the increasing and decreasing magnetic field are shown in Fig. 7 as function of temperature. The difference in g-factors
Fig. 7. Temperature dependencies of g-factor of K2Feþ3C60.
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Fig. 8. Magnetization curve for K2Feþ3C60 at T ¼ 4:2 K in low magnetic field; 1– 3 shows the sequence of magnetization loops. Fig. 10. Raman spectra of fullerides measured at room temperature.
measured in different direction of the field sweeping can be explained by hysteresis of magnetization as well as by redistribution and reorientation of particles in the powder. The width of ferromagnetic resonance curve is determined by relaxation of total magnetic moment. In our case, the width is larger for the sample, which was obtained from the Feþ2, then for the sample obtained from Feþ3. 3.4. Magnetization Typical dependences of magnetic moment on applied magnetic field is shown on Fig. 8 for K2Feþ3C60 fulleride in small applied field. At small variation of magnetic field magnetic moment of the sample decreases with an increase of applied field and vice versa. Observed magnetization curve is typical for superconductor except non-zero magnetic moment at zero applied fields. The letter is the manifestation of magnetic ordering. In high magnetic field, a magnetization curves typical for ferromagnetic was
observed (Fig. 9). Magnetization increases strongly with an increase of applied field. A residual magnetization remains in the sample. Hence, ferromagnetic ordering and superconductivity coexist in investigated fullerides. 3.5. Raman scattering Raman spectra measured on K3C60 exhibits peaks at the positions close to the positions known from literature [21]. In alkali-metal doped fulleride C60 ions line Ag(2) can be used for determination of charge state of C60 molecule [18]. A part of the Raman spectrum near Ag(2) line of K3C60, K2Feþ2C60, K2Feþ3C60 and KFeþ2 2 C60 is shown in Fig. 10. It is seen that the line position is strongly correlated with the state of Fe in initial salt used for synthesis. The position{NBsp] and shape of Ag(2) line in K2Feþ3C60 is almost the same as in K3C60, while in K2Feþ2C60 and KFeþ2 2 C60 peak is shifted towards larger values of Raman shift and the line is asymmetrically broadened. Moreover, the shape of the line in K2Feþ2C60 and KFeþ2 2 C60 is very similar.
4. Conclusions
Fig. 9. Magnetization curve for K2Feþ3C60 measured at T ¼ 4:2 K in high magnetic field.
It was demonstrated by X-ray diffraction that new method of synthesis can be applied to preparation of heterometallic fullerides. The obtained samples have fcc lattice. Mo¨ssbauer spectroscopy confirms intercalation of metal atoms in fullerides lattice. Dc magnetization measurements showed that synthesized fullerides based on iron posses ferromagnetic properties. Ac susceptibility data show that fullerides with composition K2MC60 (M ¼ Feþ2, Feþ3, Niþ2, Cuþ2) are superconductors with TC ¼ 13:9 2 16:5 K, this is also confirmed by dc-magnetization.
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Acknowledgements The work was supported by RFBR (grants 02-03-32575 and 03-03-06354). V.G.K is grateful to Alexander von Humboldt Stiftung for financial support.
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[10] M. Baenitz, et al., Solid State Commun. 96 (1995) 539. [11] Y. Iwasa, H. Hayashi, et al., Phys. Rev. B 54 (1996) 14960. [12] A.R. Kortan, N. Kopylov, E. Tzdas, Recent Advances in the Chemistry and Physics of Fullerenes and Related Materials, vol. 3, The Electrochemical Society, 1997, p. 423. [13] E. Tzdas, A.R. Kortan, et al., Nature 375 (1995) 126. [14] X.H. Chen, G. Roth, Phys. Rev. B 52 (1995) 15534. [15] B.M. Bulychev, V.I. Privalov, A.A. Dityat’ev, Russ. J. Inorg. Chem. 45 (2000) 931. [16] V.I. Goldanskii, R.H. Herber, Chemical Applications of Mo¨ssbauer Spectroscopy, 1968, p. 503 (Chapter 1). [17] J. Robert, P. Petit, T. Yildirim, J.E. Fisher, Phys. Rev. B 57 (1998) 1226. [18] C.A. Reed, R.D. Bolskar, Chem. Rev. 100 (2000) 1075. [19] S.S. Eaton, A. Kee, R. Konda, G.R. Eaton, P.C. Trulove, R.T. Garlin, J. Phys. Chem. 100 (1996) 6910. [20] A.G. Gurevich, Magnetic Resonance in Ferrites and Anti Ferromagnetics, Nauka, Moscow, 1973. [21] P. Zhou, K.-A. Wang, P.C. Eklund, G. Dresselhaus, M.S. Dresselhaus, Phys. Rev. B 48 (1993) 8412.
Heterometallic fullerides of Fe and Cu groups with the composition K2MC60 (M ¼ Feþ2, Feþ3, Coþ2, Niþ2, Cuþ1, Cuþ2, Agþ1) B.M. Bulycheva, R.A. Luninb, A.V. Krechetovb, V.A. Kulbachinskiib,*, V.G. Kytinb, K.V. Poholoka, K. Lipsc, J. Rappichc a
b
Department of Chemistry, University of Moscow State, GSP-2, Moscow 119992, Russia Department of Low Temperature Physics, Faculty of Physics, Moscow State University, GSP-2, Moscow 119992, Russia c Abteilung Silizium-Photovoltaik, Hahn-Meitner-Institute, D-12489 Berlin, Germany Accepted 15 October 2003
Abstract Heterometallic fullerides with composition K2MC60, synthesized by exchange chemical reaction of K5C60 or K4C60 with chlorides of metals Fe and Cu groups have been investigated by X-ray diffraction, magnetic resonance, Raman and Mo¨ssbauer spectroscopy. Magnetization and susceptibility measurements have also been carried out. Metal chlorides from Fe and Cu groups enable to cover the whole range of electronic configuration of metal from d5 to d10. Heterometallic fullerides with M ¼ Cuþ2, Feþ2, Feþ3 and Niþ2 appeared to be superconductors with Tc ¼ 13:9 – 16:5 K. Ferromagnetism and superconductivity coexist in investigated fulleride K2Feþ3C60. q 2003 Elsevier Ltd. All rights reserved. Keywords: A. Fullerides; A. Magnetic materials; B. Chemical synthesis; D. Superconductivity
1. Introduction Since the discovery of the fullerene C60 [1], a lot of its compounds revealed a wide variety of interesting physical properties. The most fascinating of them was the discovery of superconductivity in alkali doped fullerite [2]. The superconducting properties of fullerides detected for homo and heteronuclear compounds of alkali metals [2 –6] were discovered subsequently, for alkali-earth and some rearearth metals [7 – 14]. The intercalation of multivalent metallic atoms in fullerite lattice voids causes the consecutive filling by electrons of t1u sublevel and higherlying t1g sublevel of a fullerene molecule. Trending efforts on synthesis of heterometallic fullerides with composition K2MC60, we chose metal chlorides from Fe and Cu groups as an objects of further investigations. These chlorides allow covering the whole range of electronic configuration of metal from d5 to d10. Here we present measurement of X-ray diffraction, magnetic resonance, Raman and Mo¨ssbauer spectroscopy in the new heterometallic fullerides. In order to determine * Corresponding author. Tel.: þ 7-95-939-1147; fax: þ7-95-932-8876. E-mail address: [email protected] (V.A. Kulbachinskii). 0022-3697/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2003.10.064
critical temperature of the superconducting transition the ac susceptibility measurements were carried out. The new method of synthesis was developed, based on homogeneous –heterogeneous oxidation –reduction reactions of potassium fullerides with Feþ2, Feþ3, Coþ2, Niþ2, Cuþ1, Cuþ2 and Agþ1 chlorides in organic solvents at temperature T , 340 K. For the samples with the assumed composition K2FeC60 our Mo¨ssbauer spectroscopy studies indicate the presence of Fe atoms in the crystal lattice of heterometallic fulleride. In addition, for these samples the electron spin resonance (ESR) data are presented.
2. Experimental The potassium fulleride was prepared by reaction of fullerite with the calculated quantity of metal in the environment of toluene at 120 – 130 8C [15]. The synthesis of initial alkali metal fullerides, the elimination of toluene, the mixing of fulleride with a suspension of metal halogenide and the realization of an exchange reaction, the drying and prepacking of heterometallic fullerides in ampoules for different physical and spectroscopic investigations were carried out at vacuum in all-soldered glass
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facility. As an example, K2Feþ3C60, KFeþ2 2 C 60 and K2Niþ2C60 were synthesized using following chemical reactions K5 C60 þ Feþ3 Cl3 ! K2 Feþ3 C60 þ 3KCl K5 C60 þ 2Feþ2 Cl2 ! KFeþ2 2 C60 þ 4KCl K4 C60 þ Niþ2 Cl2 ! K2 Niþ2 C60 þ 2KCl The mixture of potassium fulleride and metal chloride is placed for 15 day in the oven with temperature 65 – 75 8C and periodically intermixed by shaking. After the end of reaction, the solvent is removed from the reactor and the precipitate is dried in vacuum. For different measurements, the dry powdered agent was placed in ampoule, which was soldered after filling by helium at pressure 350 –400 mm of mercury. The reactions were controlled by X-ray method. The X-ray data of fullerides had been recorded by photomethod with Ginie chamber (radiation lCu Ka) and listed in Table 1. As the reference, the data for K3C60 synthesized by method [15] is also shown. According to the X-ray data in all obtained precipitations the phases of metals, free fullerite and initial substances are absent. As the main phases the one of two phases of potassium halogenides (KCl or KI) are registered. This fact indicates an execution of an oxidation –reduction reaction. As it is seen in Table 1 parameter a of fcc lattice of heterofullerides less than the value of a in K3C60, synthesized under the same conditions. It seems to be reasonable because the dimensions of ions for metals of Fe and Cu groups are less, hence metals were intercalated to fulleride lattice. Laser induced ion mass spectrometry analysis was made by LAMMA-1000 device (Germany, Leybold AG) using Qswitched Nd-YAG laser (l ¼ 266 nm, t ¼ 8 2 10 ns) with power density on the sample surface 108 – 1011 W/cm2. According to these data, the structure of C60 molecule in heterofulleride is the same as in the host potassium fulleride. In the present work, we used the ac susceptibility measurements to determine the critical temperature of the superconducting transition. The samples were soldered inside the glass ampoules, thus the low frequency inductive method of measurements appeared to be very
suitable. This method is based on the Meissner– Ochsenfeld effect. The measurements were carried out at the frequency f ¼ 22:7 Hz. The magnitude of the applied magnetic field B ¼ 0:5 mT. This signal is detected by ‘Lock in’ amplifier and for control is displayed on the oscilloscope. The magnetic susceptibility x was measured in the temperature range 4:2 , T , 28 K. Magnetic resonance was studied in X-Band (9 GHz) Bruker Elexsys 580 ESR Spectrometer. A gas-flow Oxford Cryostat was used to control the temperature in the range 5 – 300 K. Raman spectra were measured by Perkin Elmer Raman Spectrometer at room temperature. For excitation 632.7 nm red line of He – Ne laser was used. Each spectrum was accumulated during 300 s. Magnetization curves were recorded by vibrating sample magnetometer EG&G PRINCETON APPLIED RESEARCH (USA), model 155, equipped by gas-flow cryostat.
3. Results and discussion 3.1. Ac-susceptibility The temperature of superconducting transition is determined as the onset of the transition. Fig. 1 shows the dependence of the magnetic susceptibility on temperature for fullerides with the composition K2MC60 (M ¼ Fe, Ni, Cu). As the reference sample K3C60 also is shown. Critical temperatures TC are listed in Table 1. Heterometallic fullerides with composition K2MC60 (M ¼ Agþ1, Cuþ1, Coþ2) and KMþ2 2 C60 (M ¼ Co, Ni, Fe) are not superconductors. The magnetic susceptibility data of K2CoC60 and KCo2C60 shows that these samples are paramagnetics down to 4.2 K. Magnetic susceptibility measurements showed that exchange reaction of K3C60 with Cuþ1 Cl and Agþ1 Cl did not produce superconducting heterometallic fullerides. This reaction reduces metal down to zero-valence state (Table 1)
Table 1 Assumed composition Tc (K) Electron configuration Lattice parameter ˚) of fullerides of metal ions a (A K3C60 K2Agþ1C60 K2Cuþ1C60 K2Cuþ2C60 K2Feþ2C60 K2Feþ3C60 KFeþ2 2 C60 K2CoC60 KCo2C60 K2NiC60 KNi2C60
18.5 – – 14.5 16.5 15 – – – 13.9 –
4d10 3d10 3d9 3d5 3d6 3d6 3d7 3d7 3d8 3d8
14.3110(5) – – – 14.24(6) 14.23(2) 14.28(2) 14.264(4) – 14.244(3) –
Fig. 1. Temperature dependence of the magnetic susceptibility of samples with the composition K2MC60 (M ¼ Fe, Ni, Cu). K3C60 is shown as the reference.
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and produces its own phase. For example, in the reaction of K4C60 with CuCl2 the fulleride K2Cuþ2C60 was synthesized. Despite the small size of diameter d of the copper ion Cuþ2 ˚ ), K2Cuþ2C60 is the superconductor with the (d ¼ 0:69 A critical temperature Tc ¼ 14:5 K. Thus, not filled d-shell in intercalated metal leads to superconductivity of fullerides in contrast to f-elements, for which superconductivity was observed in fullerides with filled f-shell. Indeed in the ˚ ), reactions of potassium fullerides with Feþ3 Cl (d ¼ 0:64 A þ2 þ2 ˚ ˚ Fe Cl (d ¼ 0:76 A), and Ni Cl (d ¼ 0:72 A) superconducting materials with composition K2MC60 were synthesized with Tc ¼ 15 2 16 K. 3.2. Mo¨ssbauer spectroscopy Mo¨ssbauer spectroscopy is a powerful tool in the determination of the valence of Fe and its positioning in the lattice [16] The Fe nuclei can experience a magnetic hyperfine field leading to a sextet in the observed Mo¨ssbauer spectrum (Fig. 2a). This spectrum is observed in to the fulleride with the expected composition KFeþ2 2 C60. The measurements were carried out at room temperature T ¼ 298 K. The parameters of sextet are isomer shift IS ¼ 0.01(0) mm/s, internal effective magnetic field Heff ð0Þ ¼ 323:9ð1Þ kOe, and an effective quadrupole moment which is equal to zero. It means that most iron in this sample is a pure a-Fe. However, a feature indicated in Fig. 2a shows the presence of some iron intercalated into fulleride lattice. Mo¨sbauer spectrum is very similar for both K2Feþ2C60 (Fig. 2b) and K2Feþ3C60 fullerides (Fig. 2b). The presence of two significant peaks (marked in Fig. 2b by two solid lines), and the width of the signal G ¼ 1:5 mm/s means that the spectrum consists of two different signals. The first signal has IS ¼ 0.09 mm/s and G ¼ 0:7 mm/s, and the second’s IS ¼ 0.5 mm/s and G ¼ 0:7 mm/s. Large width of each line means that there is quadrupole splitting of the signals ( D ¼ 0:71 mm/s for the first signal and D ¼ 0:86 mm/s for the second). The resulting fitting lines are plotted in Fig. 2b by two solid Lorentzian curves [16]. The isomer shift of the first signal is typical for Fe0 and IS of the second one is very close to Feþ3. This result is applicable also for the fulleride with the assumed
Fig. 2. Mossbauer spectra: (a) a-Fe and (b) K2Feþ2C60. Solid lines are Lorentzian fitting to the experimental data. Two short vertical lines indicate positions of two peaks of different signals.
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composition K2Feþ3C60. So, we suppose that iron intercalates potassium fulleride with valences 0 and þ 3. Quadrupole splitting can be attributed to the presence of Fe in fullerides lattice at minimum of two inequivalent positions. 3.3. Magnetic resonance The typical magnetic resonance spectra at 15 K for K3C60 is shown in Fig. 3. The spectrum is asymmetric and can be fitted with two Lorentzian lines. The width of lines increases with the increase of temperature, while the amplitude of the lines decreases. The g-values of both lines are almost temperature independent. The large fluctuations of the background signal were observed in resonance curve at temperatures below 19 K. The fluctuations vanished at temperatures higher 19 K. Magnetic resonance in K3C60 is ESR [17]. ESR signal is 32 32 due to conducting electrons, C32 60 , C60 –O– C60 and other 32 anions [18,19]. The localized anions (mainly C32 60 – O – C60 in K3C60) give narrow line with g-value about nearly two [19]. It is reasonable to associate broad line observed in our measurements with the signal from conducting electrons, although a small contribution from localized C32 60 anions cannot be excluded. The narrow line can be associated with 32 localized C32 60 – O – C60 anions which are weakly connected with crystalline lattice. The double integrated ESR intensity is proportional to the paramagnetic magnetization or paramagnetic susceptibility of the sample [17]. In K3C60 conducting electrons obey Fermi statistics, while localized C60 anions obey Boltzmann statistics. Therefore, contribution to the magnetization from conducting electrons is almost temperature independent, while the contribution from C60-anions follows Curie –Weis law. The double integrated intensity of magnetic resonance is plotted in Fig. 4 as a function of temperature. Solid line represents the fit of the dependence by the sum of Curie – Weis law and constant. The model
Fig. 3. ESR spectrum of K3C60 recorded at 15 K. Points are experimental data. Solid line is the best fitting of the experimental spectrum by two Lorentzian lines.
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Fig. 4. Double integrated intensity of ESR signal from K3C60 sample as a function of temperature.
function describes an experimental dependence with a good agreement. This gives additional evidence that ESR signal comes from conducting electrons and localized anions. Typical spectrum of magnetic resonance for K2FeC60 is shown in Fig. 5a for a sample with Feþ3. For reference, the resonance curve for K3C60 is shown in Fig. 5b. The resonance curves have almost ideal Lorentzian shape and are essentially broader in Fe-containing compound than in K3C60 (Fig. 5). The amplitude of curve decreases with the increase of temperature, while the width of the resonance increases (Fig. 6). The position of the resonance is sensitive to the sweep range and sweep direction of magnetic field. The fluctuations of the background signal were observed in K2FeC60 as in the case of K3C60. These fluctuations
Fig. 5. ESR spectrum of (a) K2Feþ3C60 recorded at 20 K (points are experimental data, solid line is the best fitting with Lorentzian line) and (b) K3C60 recorded at 20 K.
Fig. 6. ESR spectra of (a) K2Feþ2C60 and (b) K2Feþ3C60 recorded at different temperatures.
disappear at temperatures above the temperature of superconducting transition determined from magnetization measurements. The width of magnetic resonance measured on K2FeC60 fullerides is significantly larger, than the width of ESR curves measured on K3C60, while the amplitude of the signal is much smaller. All resonance curves measured on heterofullerides containing Fe can be very well fitted by one Lorentzian line. If the observed resonance is ESR, one should assume the same time of spin relaxation and same gfactor values for all spins contributing to the observed signal 32 32 (conducting electrons, localized C32 60 and C60 – O – C60 — anions and Fe ions). This means that the intercalation of Fe causes the strong interaction between magnetic moments in fulleride. More reasonable interpretation is that the spins in fulleride are ordered and observed magnetic resonance is ferromagnetic resonance. The g-factor for the ferromagnetic resonance is often close to two [20], but sensitive to the shape of sample. Calculated g-factor values obtained for the increasing and decreasing magnetic field are shown in Fig. 7 as function of temperature. The difference in g-factors
Fig. 7. Temperature dependencies of g-factor of K2Feþ3C60.
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Fig. 8. Magnetization curve for K2Feþ3C60 at T ¼ 4:2 K in low magnetic field; 1– 3 shows the sequence of magnetization loops. Fig. 10. Raman spectra of fullerides measured at room temperature.
measured in different direction of the field sweeping can be explained by hysteresis of magnetization as well as by redistribution and reorientation of particles in the powder. The width of ferromagnetic resonance curve is determined by relaxation of total magnetic moment. In our case, the width is larger for the sample, which was obtained from the Feþ2, then for the sample obtained from Feþ3. 3.4. Magnetization Typical dependences of magnetic moment on applied magnetic field is shown on Fig. 8 for K2Feþ3C60 fulleride in small applied field. At small variation of magnetic field magnetic moment of the sample decreases with an increase of applied field and vice versa. Observed magnetization curve is typical for superconductor except non-zero magnetic moment at zero applied fields. The letter is the manifestation of magnetic ordering. In high magnetic field, a magnetization curves typical for ferromagnetic was
observed (Fig. 9). Magnetization increases strongly with an increase of applied field. A residual magnetization remains in the sample. Hence, ferromagnetic ordering and superconductivity coexist in investigated fullerides. 3.5. Raman scattering Raman spectra measured on K3C60 exhibits peaks at the positions close to the positions known from literature [21]. In alkali-metal doped fulleride C60 ions line Ag(2) can be used for determination of charge state of C60 molecule [18]. A part of the Raman spectrum near Ag(2) line of K3C60, K2Feþ2C60, K2Feþ3C60 and KFeþ2 2 C60 is shown in Fig. 10. It is seen that the line position is strongly correlated with the state of Fe in initial salt used for synthesis. The position{NBsp] and shape of Ag(2) line in K2Feþ3C60 is almost the same as in K3C60, while in K2Feþ2C60 and KFeþ2 2 C60 peak is shifted towards larger values of Raman shift and the line is asymmetrically broadened. Moreover, the shape of the line in K2Feþ2C60 and KFeþ2 2 C60 is very similar.
4. Conclusions
Fig. 9. Magnetization curve for K2Feþ3C60 measured at T ¼ 4:2 K in high magnetic field.
It was demonstrated by X-ray diffraction that new method of synthesis can be applied to preparation of heterometallic fullerides. The obtained samples have fcc lattice. Mo¨ssbauer spectroscopy confirms intercalation of metal atoms in fullerides lattice. Dc magnetization measurements showed that synthesized fullerides based on iron posses ferromagnetic properties. Ac susceptibility data show that fullerides with composition K2MC60 (M ¼ Feþ2, Feþ3, Niþ2, Cuþ2) are superconductors with TC ¼ 13:9 2 16:5 K, this is also confirmed by dc-magnetization.
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Acknowledgements The work was supported by RFBR (grants 02-03-32575 and 03-03-06354). V.G.K is grateful to Alexander von Humboldt Stiftung for financial support.
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