J. Phys. Chem. 1993,97, 5717-5721
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Electron Spin Resonance and Low-Field Microwave Absorption of Alkali-Metal-Doped C a Superconductors Farid Bensebaat and Larry Kevan' Department of Chemistry and Texas Center for Superconductivity, University of Houston, Houston, Texas 77204-5641 Received: January 20, 1993; In Final Form: March 18, 1993
Alkali-metal-doped c 6 0 superconductors formed by alkali-metal azide decomposition show a time evolution for the development of the superconducting phase after initial sample preparation which extends over about 15 h for storage at room temperature. This is studied quantitatively for K3C60 and Rb&ao, and differences are seen between these two systems which are consistent with redistribution of the alkali metal in the c 6 0 matrix accounting for the time evolution observed. This is supported by separate experiments in which the storage temperature after sample preparation is decreased to 77 K. It is interesting that electron spin resonance (ESR) signals are seen in all of these samples in both the superconducting and nonsuperconducting temperature regions and that they show time evolutions after sample preparation that are qualitatively similar to the low-field microwave absorption (LFMA) signals which characterize the development of the superconducting phase. However, a detailed comparison shows differences between the ESR signal changes with time and the development of the superconducting phase which suggests that the origins of the ESR and LFMA signals are independent. It is interesting that similar ESR changes with storage time are observed in nonsuperconducting c S 3 c 6 0 as are observed in superconducting K3C60 and Rb3C60.
Introduction The new alkali-metal-doped c 6 0 superconductors show some characteristics in common with the high-temperature oxide superconductors. For example, a strong low-field microwave absorption (LFMA) below the superconductingtransition temperatureis observed in both types of superconductors.' An LFMA signal is diagnosticfor the presence of a superconductingp h a ~ e . ~ , ~ However, a significant contrast between these two classes of superconductors is the presence or absence of an electron spin resonance (ESR) signal both above and below the superconducting transition temperature, T,. In most oxide superconductors,only weak or no ESR signals are observed above T,, and those that are observed are attributed to nonsuperconducting impurity phases.* These weak ESR signals disappear below Tcunless the amount of nonsuperconducting phase is relatively large. One exception is the N ~ I , & Q , & U O + ~superconductor in which conduction ESR signals are observed below T, in the nominally pure s~perconductor.~ But, in the alkali-metal-dopedC60 superconductors,strong ESR signals are observed which surprisingly persist even below Tc.s-9 Recently, we reported a new preparation method for alkalimetal-doped c 6 0 superconductors involving alkali-metal azides as the alkali-metal s o ~ r c e .Preparation ~ occurs in a few minutes at the metal azidedecomposition temperature. After rapid cooling to room temperature, the initially generated material is partially superconductingby LFMA characterization and shows an ESR signal. A surprising aspect is that both the LFMA and ESR intensities increase by several orders of magnitude during the day following sample preparation. In this work we study the kinetics of these increases in K3C60 and Rb3C60 and of the ESR increase in cS3c60. Interesting qualitative correlations are observed between the LFMA and ESR signals. Alkali-metal diffusion or redistribution appears to be the origin of these dramatic changes.
Experimental Seetion Alkali-metal azides were used as a source of alkali metal and heated with c 6 0 in vacuum in a 3:l molar stoichiometric ratio.
' Permanent address: Institut Charles Sadron, ICS-CRM, 6 rue Boussingault 67083 Strasbourg Cedex, France. 0022-365419312097-5717$04.00/0
Right after alkali-metaldecomposition below 500 OC, which takes only a few minutes, the product is quenched to room temperature. The whole operation lasts less than 10 min. Details of this preparation have been published.IO Both ESR and LFMA were carried out using a Bruker ESP 300 X-band ESR spectrometerwith a field modulation frequency of 100 kHz. To observe LFMA, an additional pair of coils were added to the magnet pole faces, allowing a magnetic field sweep from -40 to +40 G through zero field. The temperature was varied with an ESR 900 cryostat from Oxford Instruments. Contrary to the similar technique of magnetically modulated microwave absorption,s LFMA and ESR signals may be easily recorded during the same experiment by changing the magnetic field. The maxima of both ESR and LFMA responses are obtained at the cavity position corresponding to the maximum of the microwavemagnetic field. In this study the time dependence after sample preparation is the critical parameter, so measurements are made only at 4 and 298 K for ESR and at 4 K for LFMA. The 4 K spectra are recorded after zero-field cooling. The time of initial decomposition of the alkali-metal azide is chosen as the time origin. Between measurements the sample was kept at room temperature unless otherwise stated. RMlltS
It is important to understand the chronology of the ESR and LFMA measurements after sample preparation. The measurements are made relative to the time after sample preparation at high temperature. A typical sequence of measurements after sample preparation is as follows: first the ESR spectrum is recorded at room temperature, and then the sample is cooled in zero magnetic field to 4 K. Then the LFMA signal is recorded at 4 K for a field sweep between -40 and +40 G. After this the ESR signal is recorded at 4 K near 3365 G. The time for these three measurements is typically about 30 min, and then the sample is warmed to room temperature, which is the storage temperature before additional measurements are made. An additional set of measurements at room temperature and at 4 K are made every half-hour approximately and then at successively longer times as the storage time at room temperature increases. Thus, at short storage times the measurement time can be a significant fraction
0 1993 American Chemical Society
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Bensebaa and Kevan
The Journal of Physical Chemistry, Vol. 97, No. 21, 1993 30
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Figure 1. Evolution of the peak-to-peak ESR line width of Rb3C60 at 298 K as a function of time at 298 K after sample preparation. At 4 K the ESR line width is about 10 G and time independent. Experimental conditions are 2 mW microwave power and 1 G modulation amplitude. 100
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Figure 2. Evolution of the integrated intensity of the ESR signal at 4 K for Rb3Cso as a function of time at 298 K after sample preparation. At 298 K the integrated intensity is time independent.
of the storage time at room temperature, but as the storage time increases the measurement time becomes a negligible fraction of the storage time at room temperature. The trend versus storage time is not too exact, and the overall trend of the measurements seems most important. ESR Measurements. Figure 1 shows the ESR peak-to-peak derivative line width at room temperatureas a function of storage time after sample preparation. It is seen that the line width decreases continuously over the first 15 h of storage time in the Rb3C60system and reaches a limiting value of 10.2G. For the room temperature measurements the doubly integrated ESR intensity of the Rb3C60is time independent, which means that the total spin concentration is constant even though the line width is changing significantly. When the temperature is lowered to 4 K, an ESR signal is still seen but is much smaller. At 4 K the ESR line width is time independent, but the ESR intensity increases with increasing storage time at room temperature as shown in Figure 2. The overall increase of ESR intensity over the first 15 h of storage time at room temperature is a factor of about 18, and then this intensity plateaus. Figures 1 and 2 show that the shape of the time dependence of the ESR line width at room temperature and the ESR intensity change at 4 K are not the same even though the storage time range over which the changes are observed is
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Figure 4. ESR intensity for K3C60 at 4 K versus time at 298 K after sample preparation.
approximately 15 h in both cases. The maximum ESR intensity is of the order of 10l8spins/g. Figures 3 and 4 show similar results for the K3C60 system. Figure 3 shows a change in the ESR line width at room temperature, but there is a decided contrast to the results in Rb3C60. In the K3C60 system no change in ESR line width is observed over the first 10 h or so,and then there is a sudden drop from 12.7 to 8.2 G from about 10 to 13 h, after which the line width remains constant. The ESR intensity increase at 4 K for K3C60also shows an initial induction period over the first 3 h or so, over which no ESR intensity change is observed, followed by an increase from about rll5 h, after which the ESR intensity is constant at 4 K. Again the measurements at 4 K show no change in the ESR line width, and the measurements at room temperature show no change in the overall integrated ESR intensity or spin concentration with the storage time after sample preparation. Figures 5 and 6 show similar measurements of the ESR signal in Cs&, which is nonsuperconducting. Nevertheless, this system shows a similar ESR line width decrease at room temperature as a function of storage time at room temperature. The decrease is largely complete after about 15 h of storage time. The ESR intensity increase at 4 K shown in Figure 6 shows a significant increase from 1 to -10 h, and there seems to be some symmetry between the line width change at room temperature and the ESR intensity change at 4 K in the Cs3C60system. Note that this symmetry is not observed to the same extent in the superconducting K3C60 and Rb3C60 systems.
The Journal of Physical Chemistry, Vol. 97, No. 21, 1993 5719
Alkali-Metal-Doped C60 Superconductors
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Figure 6. ESR intensity for c S J c 6 0 at 4 K versus time at 298 K after sample preparation.
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Figure 5. ESR line width for csJc60at 298 K versus time at 298 K after sample preparation.
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Figure 7. LFMA amplitudefor Rb& at 4 K versus time at 298 K after sample preparations. Experimental conditionsare 2 mW microwave power and 10 G modulation amplitude.
LFMA Measurements. Figures 7 and 8 show the time evolution of LFMA signals at 4 K for Rb3Ca and K3C60. For the first hour or so after sample preparation of Rb3C60 a weak noisy LFMA signal is clearly seen but is quite weak. Only after 1 h or more is a clear LFMA signal observed, and it increases monotonically with storage time at room temperature to about 15 h as can be seen in Figure 7. It is interesting to compare the time evolution
Figure 8. LFMA amplitude for K J C ~at O 4 K versus time at 298 K after sample preparation. Experimentalconditions are 2 mW microwave power and 10 G modulation amplitude.
in Figure 7 of the LFMA amplitude with the time evolution in Figure 2 of the ESR amplitude at 4 K. Both curves are qualitatively similar; however, the increase in the LFMA amplitude seems to occur more rapidly than the ESR intensity. This suggests that the origin of the LFMA amplitude, which relates to the development of a superconducting phase, is independent of the changes that cause an increase in the ESR signal at 4 K. Figure 8 shows the LFMA amplitude increase with storage time at room temperature for K3Ca. This is significantlydifferent from the Rb& system. First the LFMA signalright after doping is significantly larger than in Rb&, which indicates the development of more superconducting phase in the K3C60 system. The interesting thing is that the amount of superconductingphase as measured by the LFMA amplitude remains constant in the K3C60system for about 8 h storage time and then increases relatively rapidly. The increase in the K3C60 system occurs over a relatively narrow range from about 9 to 12 h, and the increase is about a factor of 17. If one compares the time evolution of the LFMA intensity in Figure 8 with the time evolution of the ESR intensity of Figure 4, the changes are qualitatively similar, but the LFMA increases more rapidly than the ESR increases. This is the same qualitative difference found in the Rb3C60 system. There are three significant differences between the time evolution of the LFMA and ESR intensities at 4 K between the K3C.50 and Rb3C60 systems. First, the shape of the increase is different between the two superconducting systems. Second, the initial LFMA amplitude is larger in the K3c60 system than in the Rb3Csosystem by a factor of 100 or so. Third, the increase in the LFMA amplitude in the K3Ca system is a factor of 17 whereas in the Rb3C60 system it is a factor of 350. At the plateau value after about 15 h storage time in both systemsthe LFMA amplitude inK3Cmisabout3 or4 timesgreater than that in Rb3C~.However, if one compares the ESR intensity increase, the increase in the K3Ca system is a factor of 45 while the increase in the Rb3C60 system is a factor of 18. So there are significant quantitative differences between the time evolutions of the LFMA and ESR signals in these two different superconducting systems, although qualitatively the trends are similar. It should be noted that no LFMA signal is observed at 4 K for the c S 3 c 6 0 system. This is as expected since c S 3 c 6 0 has not been recorded as being superconductingwhen prepared by alkali-metal exposure to c 6 0 . However, it is interesting that similar changes in the ESR line width at room temperature and the ESR intensity at 4 K after sample preparation are observed in the CsJC60system which are qualitatively similar to those changes in the superconducting &(& and RbsC60 systems.
Bensebaa and Kevan
5720 The Journal of Physical Chemistry, Vol. 97, No. 21, 1993 301
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Figure 9. ESR line width of Rb$a at 298 K versus time at 77 K after sample preparation.
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Effect of Storage Temperature. As a working hypothesis, it is suggested that the changes in the ESR and LFMA responses with storage time at room temperature after sample preparation are likely associated with the redistribution or diffusion of the alkali metals within the c 6 0 lattice. Thus, the temperature of the storage time should affect the time evolution. This was tested by a separate preparation of Rb& in which the sample was stored at 77 K after sample preparation and between measurements of the ESR and LFMA responses at room temperature and 4 K. These results are shown in Figures 9 and 10. Figure 9 shows the decrease in the ESR line width measured at room temperature as a function of storage time at 77 K. This should be compared with Figure 1. It can be readily seen that a similar ESR line width decrease occurs but over a much longer time period, approximately a factor of 10 longer, for storage at 77 K. The time evolution of the LFMA amplitude after storage at 77 K after sample preparation is shown in Figure 10, and here the time evolution is much much slower than is shown in Figure 7 for storage at room temperature. In fact, over the first 100 h of storage time at 77 K no change in the LFMA is observed, although the LFMA amplitude is significant. After 100 h there is a slow increase in the LFMA amplitude which was followed up to 350 h, and even at that point it is clearly seen that a plateau has not yet been reached. Thus, the lower storage temperature has a significant influence on both the development of the LFMA signal with storage time and the decrease of the ESR line width at room temperature. It is also clear from comparing Figures 9 versus 1 and Figures 1Oversus 7 that the ESR signal is uncorrelated
with the LFMA signal. The differencesin time evolution between storage at room temperature versus 77 K are so different for the ESR signal versus the LFMA signal that it is clear that the two types of responses are not directly correlated with each other. It should be pointed out that it has been observed that the initial LFMA amplitude at 4 K for different preparations of Rb3C60 varies significantly as can be seen by comparing Figure 10with Figure 7. This probably reflects a difference in the degree of reaction completion to form a superconducting phase during the brief time that the azide decomposition occurs. However, it is generally found that the initial LFMA signal amplitude in the K3Cm preparation is always several times larger than the LFMA amplitude in the Rb~Csopreparations at the initial measurement time after sample preparation.
Discussion Although there are qualitative similarities between the time evolution after sample preparation for the ESR signal characteristics and the LFMA signal characteristics, a more quantitative comparison clearly shows that the origins of the ESR and LFMA signals are independent of each other and are not correlated as is expected. However, it is clear that the changes that occur with the time after sample preparation in these materials affect both the ESR and LFMA signals. To reiterate, the large difference in time evolution changes after storage at 77 K between the ESR and the LFMA signals shows that there is no correlation between them. Additionally, the time evolution of the ESR intensity at 4 K compared to the LFMA intensity at 4 K is always slower for the ESR intensity. And finally, the ESR intensity increase at 4 K always lags the development of the superconducting phase as indicated by the time evolution of the LFMA signal. So all of these factors support that the origins of the ESR and LFMA signals are quite independent of one another. A constant integrated ESR intensity at room temperature with the storage time after sample preparation means that the total spin concentration is not changing. However, the ESR line width decrease at room temperature as a function of storage time clearly suggests that structural changes are occurring in the c60 material during this time. Such a decrease seems consistent with achievement of a more uniform distribution of the alkali metal in the C ~matrix O as a function of storage time. It is also possible that M6C60 phases may be involved as intermediates. At first glance the increase in the ESR intensity at 4 K as a function of storage time seems inconsistent with the constancy of the integrated ESR intensity with storage time at room temperature. However, the total spin concentration is much lower when the temperature is lowered to 4 K and the superconducting state is achieved. We hypothesize that initially the unpaired spins are in or near superconducting regions so that the spin concentration at 4 K is low due to the loss of ESR intensity from unpaired spins in the superconducting regions. However, with increasing storage time at room temperature, we suggest that the unpaired spins which give rise to the ESR signals diverge more from the superconducting regions so that the effective spin concentration observed at 4 K increases. This also explains why the storage time dependence of the ESR intensity increase at 4 K is qualitatively but not quantitatively similar to the increase in the LFMA signal which reflects an increase in the amount of superconducting phase formed. The redistribution or diffusion of the alkali metal within the c 6 0 lattice as a function of the storage time after sample preparation seems to be a working hypothesis for explaining the increase in the amount of superconducting phase formed. It is clear that the right arrangement of the alkali metal with respect to the c 6 0 lattice is required to achieve a superconducting phase which has been identified with the stoichiometry of three alkalimetal atoms per C60 molecule and which can be understood by considering the tetrahedral and octahedral vacancies in the c 6 0
Alkali-Metal-Doped c 6 0 Superconductors crystal lattice. Such a redistribution is expected to depend upon the mass of the alkali metal, and one expectsthat a greater amount of superconducting phase would be formed in the K3Cdoversus the Rb3C60system and that the amount of superconducting phase would be formed more rapidly at a given temperature in the K3C60 system compared to the Rb& system. The initial LFMA amplitude after sample preparation and initial measurements less than 1 h after preparation always show a larger value for K3c60 than for Rb3C60. Also, the time range for which the LFMA amplitude increases at 4 K occurs over a much narrower range in K3C60than in it does in Rb& (compare Figures 7 and 8 ) . However, the apparent induction period of about 8 h or so after preparation and storage at room temperature that is observed in Figure 8 for K & ,is not well understood. One possibility is that the more easily formed superconductingregions in K3C60are generated during the azide decomposition time and subsequent cooling, and it takes some induction time for some significant rearrangement of potassium atoms before additional superconducting regions can be formed. However, when those configurations are reached, then the increase in the amount of superconductingphase occurs over a relatively narrow time range. Given the way in which the experiments had to be carried out, it is not possible to define or evaluate the kinetics of superconducting phase formation in any quantitative fashion. However this is a solid-state system, and the qualitative nature of the time evolutions that are observed suggests that some sort of inhomogeneous kinetics applies.
Conclusions The preparation of alkali-metal-doped Cm superconductors by the azide decomposition method is incompleteduring the azide decomposition time, and subsequent slower development of superconducting regions occurs over a period of about 15 h for storage at room temperature. The development of more super-
The Journal of Physical Chemistry, Vol. 97, No. 21, 1993 5721 conducting regions seems to occur over a shorter time period in K3C60 than in Rb&, which supports some sort of alkali-metal redistribution as the origin of this phenomenon. An ESR signal is also seen in both the nonsuperconducting and superconducting temperature ranges of &&and Rb3c60. It shows time evolution changes with respect to storage time after sample preparation which are qualitatively analogous to those observed by LFMA characterization of the formationof more superconductingregions. But the quantitative differences indicate that the ESR signals are unrelated to the superconducting phase. This is supported by the observation of similar ESR spectral changes in cS3c60, which is not superconducting and does not show an LFMA response.
Acknowledgment. This research was partially supported by the Texas Center for Superconductivity at the University of Houston. We thank a reviewer for the suggestion of the possible intermediacy of M6C60 phases. References and Notes (1) For a review see: Haddon. R. C. Acc. Chem. Res. 1992. 25. 127. (2) Kevan, L.;Bear, J.; Puri, M.;Pan, Z.; Yao, C. L. ACS Symp. Ser. 1988,377. 223. (3) Masiakowski, J.; Puri, M.;Kevan, L. J. Phys. Chem. 1991,95,1393. (4) Bonvalot, M.; Kevan, L. J. Phys. Chem. 1992, 96, 9992. (5) Zakhidov, A. A.; Ugawa, A.; Imaeda, K.; Yakushi, K.; Inokuchi, H.; Kikuchi, K.;Ikemoto, I.; Suzuki, S.;Achiba, Y. Solid State Commun. 1991, 79, 939. (6) Glarum, S. H.; Duclos, S.J.; Haddon, R. C. J. Am. Chem. Soc. 1992, 114, 1996. (7) Wong, W. H.; Hanson, M. E.;Clark, W. G.; Griiner,G.;Thompon, J. D.; Whetten, R. L.;Huang, S.-M.; Kaner, R. B.; Diederich, F.;Petit, P.; Andrb, J.-J.; Kolczer, K. Europhys. Left. 1992, 18, 79. (8) Bensebaa, F.;Xiang, B.; Kevan, L.J. Phys. Chem. 1992,96, 10258. (9) Byszewski, P.; Stankowski, J.; Trybula, Z.; Kempinski, W.; Zuk, T. J . Mol. Strucf. 1992, 269, 175. (10) Bcnsebaa, F.; Xiang, 8.; Kevan, L.J. Phys. Chem. 1992,96,6118.
