J. Phys. Chem. 1992, 96,6118-6120
6118
A New Preparation Method for Superconducting Alkali-Metal-Doped Fullerenes: Comparison of Ac Susceptibility and Low-Field Microwave Absorption Characterization Farid Bensebaa: Bosong Xiang, and Larry Kevan* Department of Chemistry and Texas Center for Superconductivity, University of Houston, Houston, Texas 77204-5641 (Received: April 3, 1992; In Final Form: June 8, 1992)
Alkali-metal-dopedfullerenes are prepared by a rapid, new method employing alkali-metal azides. By means of ac susceptibility and low-field microwave absorption (LFMA), we have investigated the superconducting phases in K- and Rb-doped Cso With Rb doping, the transition to the superconducting state takes place at T,= 38 K when observed by LFMA. The LFMA signal shows significant hysteresis behavior. During a day following the doping process, the LFMA for nominal composition Rb3C60is found to be drastically time dependent. Measurements by ac susceptibility, on the same sample, show an onset at 34 K which is lower than the Tcobtained by LFMA but higher than the highest value reported for Rb3Csoby ac susceptibility. This could indicate the presence of another superconducting phase. At 12 K the superconducting fraction is found to be about 95% by ac susceptibility so the material formed by this new preparation method is almost entirely superconducting.
Introduction The recent discovery of superconductivity in doped fullerene' has led to renewed interest in the physics and chemistry of organic superconductors.2 Successively,superconducting transition temperatures (T,) have been reported in potassium-doped Cm at 18 K,I in rubidium-doped Cm at 28, and 30 K,4 and in cesium-doped Cm at 30 Ka5More recently, a T, of 48 K has been reported for a mixture of Cm and C70 doped with RbTl alloy6 by low-field microwave absorption (LFMA) and superconducting quantum interference device (SQUID) measurements. However, the authors of ref 6 could not repeat their enhanced Tc in Rb/Tl/C60 vs Rb3C60by SQUID magnetic s~sceptibility.~ Different groups have reported sometimes different values of T, for the same nominal composition. For K3Cmvalues of T, = 218 and 26 K9 have been reported. For Rb,C, besides T, = 25,1° 28,5 and 30 K,4JoJ1a value of 34 K6 was also found. This represents a rather large spread. In cesium-doped fullerene both the presence' and absence" of superconductivityhave been claimed. However, in the former, the superconductingfraction is very small (1%).
Factors that may contribute to this spread in T, include (i) a difference in the purity and/or origin of the alkali metal used, (ii) the resulting homogeneity of the preparation, (iii) the high sensitivity of these compounds to ambient air,l and (iv) the different characterization techniques, mainly susceptibility and microwave absorption. As pointed out by Iqbal et a1.,6 an increase of the doping temperature in KTICm can increase the T, from 17.613to 23 K. The difference in the preparation procedure may also result in different fractions of superconductivity in the compound. Fractions of 1%' and 40%11for K3C, and 7%" and nearly lOO%I4for Rb3Cmhave been reported. Most M,Cm (where M = alkali metal) compounds are prepared by direct reaction of pure alkali-metal vapor with Cso for typically For x = 0 or 6 one obtains a molecular 24 h or more.1*3*4*8-10 insulator, but for x 3 a superconducting phase is obtained." Several studies on binary-alloy-doped c 6 0 have also been rep~rted.~*I~ Often these binary alloy preparations show an increase in T, by a few kelvin in comparison with single-alkali-metal-doped
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c60.
Here we use a new approach involving alkali-metal azides as the source of alkali metal and heating the a z i d e + , mixture for only about 5 min at high temperature to prepare superconducting polycrystalline samples with nominal compositions of K3C, and Rb3Cm. These materials have been characterized and compared by ac susceptibility and LFMA. Of particular interest is that the Rb3C60preparation shows about 95% superconducting material with T, = 38 f 1 K. Permanent address: Institut Charles Sadron, ICS-CRM, 6 rue Boussingault, 67083 Strasbourg Cedex, France.
0022-365419212096-6118$03.00/0
Experimental Section Smple Prepamtion, The fullerene C, (99.9% purity), obtained from Texas Fullerene Corp., used during this work shows no electron spin resonance (ESR) signal at room temperature. RbN3 was made by neutralization of rubidium hydroxide solution with a 3% solution of hydrazoic acid (HN3).I5 RbN, and Cso powders in the mole ratio of 3:l are mixed in a 2-mm4.d. by 3-mm-0.d. Suprasil quartz tube. After connecting the tube to a vacuum line and pumping it at 0.5 mTorr for 1 h, we heated the sample under vacuum in a homemade vertical tube furnace at 300 OC for 30 min to remove absorbed water and other organic impurities. Then the heating temperature is i n c r d to about 500 OC to decompose RbN3. The azide decomposition was shown by a pressure increase monitored by a vacuum gauge. After the decomposition was complete in a few minutes the pressure decreased, and the sample was cooled to room temperature and sealed in the quartz tube. The decomposition of RbN3 in the mixture with Cm took about 5 min. Here, we report measurements on K3C, and Rb3Cmwhere the molecular compositions are nominal as based on the starting mixture mole ratio. CharacterizationMethods. The ac susceptibility is measured by means of a mutual inductance techniqueI6 in a superconducting characterization cryostat from APD, Inc. The sample is placed inside two concentric coils. An ac voltage of 25 Hz and 20 mV rms (generating a 0.18-G rms magnetic field) is fed to the outer primary coil, and any difference between the two inner secondary coils, one of which contains the sample, is detected by a lock-in amplifier (PAR Model 5210). For accurate measurements the primary coil is moved up or down to create a zero voltage between the two secondary coils in the absence of a sample. A Displex CS202 JouleThomson refrigerator achieves temperatures down to 12 K by using a closed cycle helium system. A digital temperature controller (SI Model 9650) with a calibrated silicon diode was used to monitor and control temperature. The silicon diode sensor position was movable and positioned next to the sample location. Background measurements versus temperature with an empty sample tube were subtracted from measurements of the tube containing a sample. By using the two-phase output of the lock-in amplifier, voltage signals associated with the in-phase x' (real) and out-of-phase x" (imaginary) components of the complex susceptibility are measured for a reference frequency of 25 Hz. The same samples are measured by LFMA and ac susceptibility. Since the sensitivity of ac susceptibility is lower than that of LFMA, the ac susceptibility shows a low signal-to-noise ratio. LFMA is carried out using a Bruker ESP 300 X-band ESR spectrometer with a field modulation frequency of 100 W z . An additional pair of coils are added to the magnet pole faces to allow a magnetic field sweep from -40 to +40 G through zero field. Temperature variation was achieved by an Oxford ESR 900 cryostat. The temperature was monitored with a separate cali0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 15, 1992 6119
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Temperature (K) Figure 1. Temperature dependence of the real part of the ac susceptibility of Rb3CWprepared by metal azide decomposition measured 7 days after preparation.
TABLE I: Normalized Values of the LFMA Amplitude of Rb3C0 versus the Time after Doping for Various Temperatures temp, K 4 10 20 30 35 38
2h 0.004 0.014 0.016 0.03 0.01 0
24 h 10 9.5 12.3 26 6 0
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brated Au-Fe/chromel thermocouple system positioned next to the sample tube and away from the heater of the Oxford cryostat. The thermocouple and its readout were calibrated at the boiling points of liquid helium at 4.2 K and liquid nitrogen at 77 K. After inserting the tip of the thermocouple in a 1-mm4.d. by 2-mm-0.d. Suprasil quartz tube, we introduce this thermocouple tube adjacent to the sample tube inside the Oxford cryostat. This should give accurate temperatures of the sample which is also in a quartz tube. Our measurements are reproducible within 1 K.
Results Ac Sllooeptibility. Figure 1 shows the temperature dependence of the real part of the ac susceptibility for Rb3Cb0. The sharp decrease of the real susceptibility below 30.5 K following an onset around 34 K is strong proof for the presence of superconductivity. The observation of a two-step drop within onset at 34 K is difficult to understand. The absolute susceptibility was calibrated with Gd2(S04)3-8H20which has x‘ = 0.6692 (SIunits) at 293 K.” At 12 K the x’ of Rb3Csowas near its minimum plateau and is measured as 4.95 (SIunits) by considering the powder as a set of spheres with a demagnetization factor of one-third.I6 Within our experimental error the imaginary part x” of the ac susceptibility is not detectable above the noise level. Low-Field Microwave Absorption. LFMA data are shown in Figures 2 and 3 for K3CWand Rb3Cm The LFMA response shows some hysteresis between forward and backward field sweeps. Since the LFMA response is quite sensitive to small amounts of superconducting material, it is a sensitive method to assess any time dependence of the superconducting amount in the samples after preparation by the alkali-metal azide method. Table I shows rather remarlable results for Rb3Csa. Recall that the entire sample preparation process takes less than 1 h, including cooling to room temperature after about a 5-min azide decomposition. An LFMA signal is seen in the initial material at 2 h after preparation which indicates some superconducting material. However, this signal increases in intensity by about lo3 times within 24 h after which it appears to be stable for at least 2 months. The susceptibility in Figure 1 is measured on Rb3Cso7 days after preparation and indicates that about 95% of the material is superconducting. The
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2. LFMA response versus temperature for K3Ca showing forward and backward magnetic field sweeps as shown by the arrows. Experimental conditions are 2-mW microwave power and 5-G modulation amplitude. F i i
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Figure 3. LFMA response versus temperature for Rb3CWshowing forward and backward magnetic field sweeps as shown by the arrows. Experimental conditions are 2-mW microwave power and 10-G modulation amplitude.
time-dependent results indicate that the sample preparation procedure immediately forms only a small amount of superconducting material. Presumably the Rb atoms have not diffused
6120 The Journal of Physical Chemistry, Vol. 96, No. 15, 1992
prolonged heating is unnecessary. The difference of about 4 K in the value of T,measured by LFMA and ac susceptibility may result from the greater sensitivity of LFMA. It is reasonably well established that the principal origin of the LFMA signal is the intergrain and intragrain coupling via Josephson junctions.18-22The increase of T, by 4-5 K for Rb3Cmcompared to other literature values observed by LFMA and ac susceptibility may possibly be attributed to our new preparation method and may indicate the formation of a new superconducting phase.
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Temperature (K) Figure 4. Temperature dependence of the LFMA amplitude for Rb3Ca 2 h, (0)24 h, (+) 48 prepared by metal azide decomposition after (0) h, (A)48 h plus shaking, and (w() 48 h plus shaking plus 48 h. The solid lines are only guides to the trends.
to their optimum location. However, they seem to be able to diffuse somewhat in Cw at room temperature within 24 h to reach an optimum location and convert the Cm almost entirely to the superconducting Rb3C6,, phase. We have also observed for K3Csoa similar time dependence in the LFMA amplitude for K3Cso. It seems that the diameter of the alkali-metal atom does not change the kinetic of diffusion for K and Rb in Cm crystallite. Figure 4 shows the LFMA temperature dependence for Rb3Cso. The onset temperature for an LFMA response is 38 K. The shape of this temperature dependence is similar to that for the hightemperature cuprate superconductors.18 It is striking that the shape can be changed and the maximum shifted to lower temperature by about 10 K just by shaking vigorously. Shaking apparently changes the packing density of the powder sample in the tube which affects the LFMA intensity at a given temperaturea8 A similar shaking effect on the LFMA of K3Cmhas been observed.8
Discussion The observation of an LFMA signal below a certain temperature and the temperature dependence of the ac susceptibility unequivocally shows the presence of superconductivity in Rb& and K3Cm obtained by our synthesis method. Based on our stoichiometric composition and on earlier ~ t u d i e s , ~the ' J ~ phase x 3 appears to be the origin of the observed superconductivity. The time dependence of the LFMA intensity during about 24 h after the doping is probably due to the diffusion and redistribution of Rb atoms within the Cso. Another possibility is the slow kinetic3 of the Rb reaction." Several groups have already reported that thermal annealing improves the quantity of the superconducting phase.6p8J1 The process of the annealing is believed to mainly homogenize the doping throughout the sample. In previous preparation methods where an alkali metal is heated with Cm, it is expected that the metal atoms have to diffuse into the Cm particles or crystals and give rise to a gradient in the doping level. In this work, the nitrogen evolution in the azide decomposition appears to enhance the incorporation of the metal atoms into the Cm particles. Although recent data show that, following an alkali-metal preparation, longer annealing time at higher temperature increases the fraction of the superconducting phaseYz1 we have shown in the metal azide decomposition method that
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Conclusions A new method for preparing superconducting M3Cq compounds (M = alkali metal) involving metal azide decomposition has been described. This method involves only brief heating at high temperature in contrast to other methods. After the metal azide decomposition, room temperature annealing for 24 h produces Rb3Cm with a larger superconducting fraction and as high or higher T, than generated by other methods. Acknowledgment. This research was supported by the Texas Center for Superconductivity at the University of Houston under Grant MDA972-88-G-0002 from the Defense Advanced Research Projects Agency and by the State of Texas.
References and Notes (1) Hebbard, A. F.; Rwseinsky, J. J.; Haddon, R. C.; Murphy, D. W.; Glarum, S.H.; Palstra, T. T. M.; Ramirez, A. P.; Kortan, A. R. Nature 1991, 350, 600. (21 Williams. J. M.: Schultz. A. J.: Geiser. U.: Carlson. K. D.: Kini. A. M.;Wang, H. H.; Kwok, W.-K.; Whangbo, M.-H.; Schirbcr, J. E.'Sciences 1991, 252, 1501. Jerome, D. Science 1991, 252, 1509. (3) 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. Rev. Lerr. 1991, 66,2830. (4) Holczer, K.; Klein, 0.;GrUner, G.; Thompson, J. D.; Diederich, F.; Whetten, R. L. Phys. Rev. Lett. 1991, 67,271. (5) Kelty, S. P.; Chen, C.-C.; Lieber, C. M. Nature 1991, 352, 223. (6) Iqbal, Z.; Baughman, R. H.; Ramakrishna, B. L.; Khare, S.;Murthy, N. S.;Bornemann, H. J.; Morris, D. E.Science 1991, 254, 826. (7) Iqbal, Z.; Baughman, R. H.; Ramakrishna, B. L.; Khare, S.;Murthy, N. S.;Bornemann, H. J.; Morris, D. E. Science 1992,256,950. (8) Zakhidov, A. A.; Ugawa, A.; Imaeda, K.; Yakushi, K.;Inokuchi, H.; Kikuchi, K.; Ikemoto, I.; Suzuki, S.; Achiba, Y. SolidState Commun.1991, 79, 939. (9) Sungi, F.; Xing, 2.;Wu, E.; Jishi, F.; Jinchang, M.; Zhennan, G.; Jiuxin, Q.; Xihuang, Z.; Zhaoxia, J.; Bo, Y. Solid State Commun.1991,80, 639. (10) Glarum, S.H.; Duclos, S. J.; Haddon, R. C. J. Am. Chem.Sw.1992, 114, 1996. (11) Holczer, K.; Klein, 0.; Huang, S.-M.; Kaner, R. B.; Fu, K.-J.; Whetten, R. L.; Diederich, F. Science 1991, 252, 1154. (12) McCauley, Jr., J. P.; Zhu, Q.;Coustel, N.; Zho, 0.;Vaughan, G.;
Idziak, S. H. J.; Fischer, J. E.; Tom, S . W.; Groski, D. M.; Bykovetz, N.; Lin, C. L.; McGhie, A. R.; Allen, B. H.; Romanow, W. J.; Denestein, A. M.; Smith, A. M. 111 J. Am. Chem. Soc. 1991, 113, 8537. (13) Krauss, M.; Freytag, J.; Glrtner, S.;Vieth, H. M.; KrHtschmer, W.; Liiders, K. 2.Phys. B 1991, 85, 1. (14) Chen, C.-C.; Kelty, S. P.; Lieber, C. M. Sciences 1991, 253, 886. (15) Xu, B.; Kevan, L. J. Chem. Soc., Faraday Trans. 1991,87, 2843. (16) Goldfarb, R. B.;Minervini, J. V. Rev.Sci. Insrrum. 1984, 55, 761. (17) H a n d h k of Chemistry and Physics, 63rd ed.;Weast, R. C., Astle, M. J., Eds.;CRC: Boca Raton, FL, 1982; p E-1 19. (18) Puri, M.; Marrelli, S.;Cuvier, S.;Bear, J.; Kevan, L. J. Chem. Soc., Faraday Trans. 1991,87, 167. (19) Fleming, R. M.; Ramirez, A. P.;Rosseinsky, M. J.; Murphy, D. W.; Haddon, R. C.; Zahurak, S. M.; Makhija, A. V. Nature 1991, 352, 787. (20) Stephens, P. W.; Mihaly, L.; Wiley, J. B.; Huang, S.-M.; Kaner, R. B.; Diedench, F.; Whetten, R. L.; Holczer, K. Phys. Rev. B 1991, 45, 543. (21) Murphy, D. W.; Rwseinsky, M. J.; Haddon, R. C.; Ramirez, A. P.; Hebard, A. F.; Tycko, R.; Fleming, R. M.; Dabbagh, G. Physica C 1991. 185-189,403. (22) Blazey. K.W. In Earlier and Recent Aspects of Superconductivity; Bednorz, J. G., Muller, K.A., Eds.; Springer-Verlag: New York, 1990; p 262.
