l02ss
J. Phys. Chem. 1992,96, 10258-10261
Electron Spin Resonance of AlkalEMetaCDoped Fullerenes Farid Bensebaa,+ Bosong Xiang, and Larry Kevan* Department of Chemistry and Texas Center for Superconductivity, University of Houston, Houston, Texas 77204-5641 (Received: June 1 , 1992; In Final Form: September 8, 1992)
Electron spin resonance (ESR) measurements including spin susceptibility, line width, and g value are given for M3Ca nominal compositions where M is an alkali metal. During about 24 h following the alkali-metal doping, the ESR signal of Rb3Ca is found to be time dependent in the temperature range 4-290 K. At room temperature, the same time dependence is also found in K3Ca and Cs3Ca but not in Na3Ca, which is not superconducting. This time dependence of the ESR signal at room temperature appears to correlate with the development of a superconducting phase in K3Ca and Rb3Ca at lower temperature. The ESR line width 24 h after doping scales with the size of the crystal unit cell of M3Ca. For RblCa the temperature dependence of the ESR susceptibility, after 1 day or more, indicates a transition from an antiferromagnetic to a paramagnetic state around the superconducting transition temperature.
Introduction We have prepared superconducting K3Cmand Rb3Ca with a new preparation method involving an alkali-metal azide as the alkali-metalm m . l For these two nominal compositions low-field microwave absorption (LFMA) gives transition temperatures (T,) of 19 and 38 K, respectively. For Rb3Ca the amplitude of the LFMA signal increases with time after sample preparation until it becomes constant after about 24 h. After 24 h about 95%of the perfect shielding diamagnetism is found at the low-temperature plateau of the real part of the ac susceptibility. Few studies have yet been carried out on the magnetism of these material^."^ Electron spin resonance (ESR) is a powerful probe of the dynamic (g factor line width) and static (spin susceptibility) properties. In such systems it is important to consider other possible sources of ESR signals such as alkali-metal particles,6 the Cm- anion radical,' and the c 6 0 + cation r a d i ~ a l . ~ . ~ In this paper we observe ESR signals in Na3Cm K3Cm Rb3Ca, and Cs3Ca obtained by the alkali-metal azide synthesis although only K3Ca and Rb3Ca are superconducting. A time dependence of the ESR signal after sample preparation is observed except in Na3Cso. In this study we focus on Rb3Ca but also discuss the ESR signals of other alkali-metal-doped fullerenes. The temperature dependence of the spin susceptibility of Rb3Csoindicates a change from an antiferromagnetic state to a paramagnetic state around T,. The results suggest nonnegligible electron spin-electron spin interactions.1° Experimental Section The fullerene Ca used during this work is obtained from the Texas Fullerene Corp. No ESR signal is observed in a fresh Ca sample at room temperature. ESR measurements are carried out using a Bruker X-band spectrometer with a field modulation frequency of 100 kHz. The temperature variation was controlled by an Oxford ESR 900 helium flow cryostat. The maximum response is obtained for a sample positioned at the maximum of the microwave magnetic field. The spin concentration and g factor were calibrated with CuS04.5H20 and DPPH (a,a'-diphenyl-/3-picrylhydrazyl). ReSultS ESR at Room Temperature. As shown in Figure 1, there is no time dependence of the ESR signal in Cb0 doped with Na. However, a decrease of the FSR line width is observed during about 1 day for Cm doped with K,Rb, and Cs. For K3Cmand Rb3Car the time dependence of the ESR line width is similar to the intensity increase of the LFMA at low temperature.' Within an error of less than 50%. the spin concentrations seem to be time 'Permanent address: Institut Charles Sadron, ICS-CRM, 6 rue Boussingault, 67083 Strasbourg Cedex, France.
independent for all doped samples. Except for Na3Cm the doped samples exhibit broad and narrow ESR lines, but they are not resolved for K3Cm. The broad lines have g 2.003, and the MITOW lines have g 2.001 right after doping. After 1 day, only a narrow line is distinguished with g = 2.0013 f O.OOO5 (Figwe 1). The spin concentration,determined by double integration with reference to a weighted CuSO4.5Hz0 standard, is estimated to be (3 f 1)%spin per Ca for Rb3Ca and K3Cm This value agrees with reported values for K3C6Jq4and seems characteristic of alkali-metal-doped Cm. Temperature Depe&ae. The ESR and Rb3Ca was recorded between 4 and 280 K. The temperature dependence of the doubly integrated intensity is shown in Figure 2. Within experimental error the FSR intensity is time independent for T > 80 K below 80 K it is strongly temperature dependent about 24 h after doping. In Figure 3, the temperature dependences of the peak-tepeak line width at 4,32, and 57 h show a broad minimum around 100-150 K with AHp 5 G. The plot of the inverse ESR intensity as a function oftemperature shows different slopes above and below 40 K (Figure 4). This suggests applicability of a Curie law, x = C/( T - e), where C is the Curie constant and 0 is the CurieWeiss temperature. By a least-squares fit one obtains
-
-
x
x
(au) = 5.O/T
(au) = 10.5/(T
for T < 40 K
+ 45)
for T > 40 K
The Curie constant is expressed as
where No is the free spin number per mole, S is the spin quantum number, g is the g value, p e is the Bohr magneton, and k is Boltzmann's constant. Using the value of the spin concentration at room temperature No = O.03Na, where N , is Avogadro's number, the Curie constant is C = 0.1 1 (emu K mol-') between 40 and 280 K and C'= (5/lOS)C = 0.055 (emu K mol-') below 40 K. Expressing the negative Weiss temperature of -45 K by B - 2rJS(S + 1)/3k where z is the coordination number (z = 4), S= is the spin quantum number, J is the spin exchange interaction, and k is Boltzmann's constant, one finds IJ/kl 5 22 K. The temperature dependence of the spin susceptibilityis about the same from 24 h to 2 months after doping. The behavior of the inverse of the ESR susceptibility is typical of Rb3Cw samples 1 day or more after doping. The change of the temperature dependence of the ESR susceptibility near the onset temperature (-35 K) of the superconducting state is difficult to understand. Analogies with heavy-fermion"J2 and magnetic superconductorsI0 are perhaps relevant. Figure 5 shows the g value temperature dependence of Rb3Ca. At room temperature the g value is lower than that of a free electron, and it decreases continuously with decreasing temperature to g = 2.0034 at 13 K.
0022-3654/92/2096-10258$03.OO/0 0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No, 25, 1992 10259
ESR of Alkali-Metal-Doped Fullerenes K3C60
2.0039 2.0008
2.0005
60Min.
; ; 1
8oM%~
2.0006
2.0013
2.0014
4
: :17
2.0012
2.0016
A
A
Figure 1. Time dependence of the ESR signal at room temperature after doping Csowith different alkali metals: (a) Na, (b) K,(c) Rb, and (d) Cs. At the left side of each spectrum is the temperature at which the mixture was heated, the time after doping at which the spectra were recorded, and the g value respectively from top to bottom. At the right side is the line width and the relative doubly integrated intensity. In the case of RblCso and CslCw right after doping the line widths of both narrow and broad signals are given. 20 P
F
a-
4 0 1
o
4
hours
32 hours
3
15
.
10;
Note that relatively strong ESR signals are observed in Rb3Cso below its superconducting transition temperature. Apparently, the unpaired electrons are pnsmt in nmuperamducting domains of this type I1 superconductor in a mixed state. Any explanation is unclear at the moment.
Disepseion The appearance of a timedependent ESR signal in the metaldoped fullerene material indicates that charge transfer occurs between K,Rb, or Cs metal and Cm This d m not seem to be the case for Na. The time-independent ESR line observed in Na3Ca after doping is assigned either to Ca+ due to air oxidation9 or to a Na particle.6 The second possibility is considered more likely. Positive and negative g shifts from the free electron value of ESR signal have been reported in electrolyzed solution of Cso+ and Ca-, respectively? Thus,one might attribute the observed One narrow ESR signal at g > 2.0023 to the cation radical C.,' drawback of this interpretaton is why a C,+ signal with a positive g shift is not observed for Na3Cm
# A
f
hours
o
4
A
57 hours
32 hours
A A
~
0 .
a
p- a \ *
0
0
%
0
at
El
-0.
5 -
200 250 Temperature (K) 2. Temperature dependence of the ESR intensity of Rb& different times after Rb doping.
0
- ?
i
o""""""""""""""~' 0 50 loo 150
.
8
0
00000
0
A I
" " " " ' " ' " " " ' " ' " ' " "
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Bensebaa et al.
10260 The Journal of Physical Chemistry, Vol. 96, No. 25, 1992
properties in heavy-fermion superconductors.' I The magnitude of the spin susceptibility at room temperature 1*12
0.4
0.3 1
a
-50
0
50
100
150
200
250
temperature (K)
Figure 4. Evolution of the inverse of the ESR intensity in RbJCpo1 day after doping as a function of temperature. The two straight lines are those obtained by least-squares fits to the Curie-Weiss law (see text).
2.0015
1
N.
( 3 X lo4 emu mol-') is about 10 times the value from an "isotropic" noninteracting electron gas as shown below. The Pauli susceptibility from itinerant conduction electrons is described by2l xpauli FB~D(&)where D(EF)= 4 3 d 1 ) ' / ~ ( 4 / h is ~ )the density of states at the Fermi level in units of states/eV/Cm and bB is the Bohr magneton amstant. Assuming n = 0.03 spin per molecule and taking md = 4 mZ2 as a value of the effective electron mass, the Pauli susceptibility is around 4 X lo-$ emu/mol in disagreement with the assignment of the observed spin susceptibility only to conduction electrons. Thus, considering the conduction electrons in the framework of a Fermi liquid theory is not a p propriate. Instead, one can define a renormalized susceptibility based on the replacement of noninteracting fermions by interacting quasi-particlesz3so that
where U, is the mean field electronic interaction. The existence of an interaction between the nearly free electrons agrees with the high value of the reduced energy gap 2A(0) = 5.2kTcZ4 compared to the predicted one, 2A(O) = 3.52kTCwithin the BCS model of superconductivity. The importance of magnetic interactions has also been emphasized in recent theoretical calculations on doped fullerene2$and in photoemission spectroscopy measurements." Relation 2 accounts for the enhancement of the spin suscep tibility. However, this relation does not explain the change of around T, and the temperature dependence of the g value. These properties may be explained by considering the coexistence of two spin states: conduction (itinerant) and localized (fixed). A good representation of the experimental spin susceptibility is a combination of CurieWeiss localized states and Pauli-like itinerant states. One may then express the total susceptibility by the equation Xspin
= Xi + XI
(3)
where the subscripts "i" and "1" stand for intinerant and localized contributions. A possible origin of the localization is the presence of disorder. Recent band calculations demonstrated a large effect of the orientational disorder on the magnetic and electronic properties of Ca and related superconductors.26 The absence of a metallic phase above T, is highlighted by transport property measurements?' where for T > T, an increase in the resistivity with decreasing temperature was reported for K3Ca and R&CW2' This behavior was ascribed to the presence of disorder. The coexistence of an array of local moments and a sea of conduction electrons also accounts for the thermal dependence of the g factor. As proposed for copper phthalocyanine iodide, the average g factor may be expressed as by the equationZ8 g = (xl/xspih+
(Xi/xspdgi
(4)
where g,and gi stand for the g value of localized and itinerant electron spins, respectively. Thus, the increase of the g factor with decreasing temperature is due to xI. Although that this picture fits our interpretation, a temperature variation of the g value due to spin-orbit coupling may also obtain. This process was used by Pedersen et al?9 and Yafet et aL30to explain the increase of the g factor with decreasing temperature in (TMTSQ2PFs and graphite, reapectively. Rb3Cm, copper phthalocyanine iodide, (TMTSF)zPF6,and graphite are all r-electron-based systems. The indirect exchange between the localized moments via the conduction electrons is responsible for the observed narrow line widths in the high-temperature range. The change of the line width and the spin susceptibility around 40 K is related to a drop in the efficiency of this exchange interaction due to the formation of the superconducting state. This means that the conduction electrons cease to play their role as a medium for interaction between the localized spins. The electron reservoir created by doping Rb into C a may be divided into three parts. One part consists of the spin susccpubility
10261
J. Phys. Chem. 1992,96, 10261-10264
(8) Kato, T.;Kodama, T.; Oyama, M.; O W , S.;Shida,T.; Nakagawa, T.; Matsui, Y.; Suzuki, S.;Shiromaru, H.; Yamauchi, K.;Achiba, Y. Ch" Phys. Lett. 1991, 186, 35. (9) An aging effect is observed for pure C a which is revealed by an appearanceof a weak ESR signal with 1-24 line width and g = 2.0024. The ColleldoILs intensity of this signal also depends on any heat treatment. (10) Chakravarty, S.;Kivelson, S.A.; Salkola, M.I.; Tewari, S . Sclence During about 24 h the ESR signals of Rb3Ca evolve with time. 1992,256, 1306. This is suggested to be related to diffusion of the Rb atoms within (11) Stewart, G. R. Reu. Mod.Phys. 1984.56.755. the Cm crystallites. Near T, the ESR susceptibility is found to (12) Fisk, 2.;Hew, D. W.; Pethick, C. J.; Pines, D.; Smith, J. L.; Thompson, J. D.; Willis, J. 0. Science 1988, 239, 33. change from antiferromagnetic to paramagnetic behavior. The (13) Bames, S.E. Adu. Phys. 1981, 30, 801. magnitude of the ESR susceptibility is described by a two-com(14) Anderson, P. W.; W e b , P. R. Reu. Mod.Phys. 1954, 25, 269. ponent model of localized and conduction electrons. The ESR (15) Fleming, R. M.;Ramircz, A. P.; Raseinsky, M.J.; Murphy, D. W.; signals of K3Ca and Cs3Ca also show a similar timedependent Haddon, R. C.; Zahwak, S. M.;Makhija, A. V. Nature 1991, 352, 787. (16) Duclos, S.J.; Haddon, R. C.; Glarum, S.;Hebard, A. F.; Lyons, K. behavior. B. Science 1991,254, 1625. (17) Gu, C.; Stepniak, F.; Poirier, D. M.; Jost, M.B.; Benning, P. J.; Chcn, Acknowledgment. This research was supported by the Texas Y.; Ohno, T. R.; Martin, J. L.; Weaver, J. H.; Fure, J.; Smalley, R. E. Phys. Center for Superconductivity at the University of Houston under Reu. B 1992, 45, 6348 and references therein. Grant MDA972-88G-OOO2from the Defense Advanced Research (18) Kinoshita, N.; Tanaka, Y.;Tokumoto, M.;Matsumiya, S. J. Phys. Projects Agency and by the state of Texas. Soc. Jpn. 1991,60,4032. (19) Kortan, A. R.; Kopylov, N.; Glarum, S.;Gyorgy, E.M.; Ramirez, A. RWhy NO. Rb&, 137926-73-9; K3Cw 137232-17-8; C S ~ C ~ , P.; Fleming, R. M.;Thiel, F. A,; Haddon, R. C. Nature 1992, 355, 529. 140883-40-5; Na3Cw 139242-45-8. (20) Jorgensen, J. D.; Pei, S.;Lightfoot, P.; Shi,H.; Paulikas, A. P.; Veal, B. W. Physica C 1990,167, 571. Rdereaees d Notes (21) See for instance: Ashcroft, N. W.; Mermin, N. D. Solid Srare (1) Bensebaa, F.; Xiang, B.; Kevan, L. J. Phys. Chem. 1992, 96, 6118. Physics; Holt, Rinehart and Winston: New York, 1976; p 661. (2) Haddon, R. C.; Hebard, A. F.; Raseinsky, M.J.; Murphy, D. W.; (22) Holczer, K.;Klein, 0.; Grflner, G.; Thompson, J. D.; Diederich, F.; Duclos, S.J.; Lyons, K. B.; Miller, B.; Rosamillia, J. M.;Fleming, R. M.; Whetten, R. L. Phys. Rev. Lett. 1991, 67, 271. Kortan, A. R.; Glarum, S.H.; Makhija, A. V.; Muller, A. J.; Eick, R. H.; (23) Morya, T. Spin Fluctuation in Intinerant Electron Magnetism; Zahurak, S.M.;Tycko, R.; Dabbagh, G.; Thiel, F. A. Nature 1991,350,320. Springer-Verlag: Berlin, 1985. (3) Zhakhidov, A. A,; Ugawa, A.; Imaeda, K.;Yakushi, K.;Inokuchi, H.; (24) Zhang, Z.; Lieber, Ch. M.Mod.Phys. Lett. 1991, BS, 1905. Kikuchi, K.;Ikemoto, I.; Suzuki, S.;Achiba, Y. Solid State Commun. 1991, (25) Chakaravarty, M.;Gelfand, M.P.; Kivelson, S . Science 1991, 254, 79, 939. 166. (4) Glarum, S.H.; Duclos, S.J.; Haddon, R. C. J. Am. Chem. Soc. 1992, (26) Gelfand, M. P.; Lu, J. P . Phys. Rev. Lett. 1992, 66, 1050. 114, 1996. (27) Ogata, H.; Inabe,T.; Hashi, H.; Maruyama, Y.; Achiba, Y.; Suzuki, (5) Tycko, R.; Dabbagh, G.; Roaseinsky, M.J.; Murphy, D. W.; Fleming, S.;Kikuchi, K.;Ikemoto, I. Jpn. J . Appl. Phys. 1992, 31, 166. R. M.;Ramirez, A. P.; Tully, J. C. Science 1991, 253, 884. (28) Ogawa, M.Y.; Hoffman, B. M.;Lee, S.;Yudkowsky, M.; Halperin, (6) Hugha, A. E.;Jain, S.C. Adu. Phys. 1979, 28, 717. W. P. Phys. Rev. Lett. 1986,57, 1177. (7) Allemand, P. M.;Srdanov, G.; Koch,A.; Khemani; K.;Rubin, Y.; (29) Pedersen. H. J.; Scott, J. C.; Bechgaard, K.Solid State Commun. Diederich, F.; Alvarez, M.M.; Anz, S.J.; Whetten, R. L. J. Am. Chem. Soc. 1980, 35, 207. 1991, 112,2780, (30) Yafet, Y. Solid State Phys. 1963, 14, 1.
detected by ESR (3%), the second part comprises the electron pairs which participate in superconductivity,and the third part comprises electron spins arranged antiferromagnetically.
A Correlation between Proton Aff lnities and Intramolecular Hydrogen Bonds in Bifunctional Organic Compounds S . Yambe,* Department of Chemistry, Nara University of Education. Takabatake-cho, Nara 630, Japan I(. Hirao,
Department of Chemistry, College of General Education, Nagoya University, Nagoya 464- 01, Japan
and H.Wasada Department of Chemistry, College of General Education, Gifu University, Gifu 501 - I I , Japan (Received: June 17, 1992; In Final Form: September 15, 1992)
The geometries of diethers, diketones, and diamines and their protonated species are determined with the ab initio calculati~n~ of RHF/3-21G. The theoretical proton affinities (PA's) of MP2/6-31(+)G(**)//RHF/3-21G reproduce well experimental PA's. A good correlation between PA's and hydrogen-bond angles is found. As the alkyl size increases, the angles and ring strains at the sp3 carbons become larger, which leads to a "saturation" of PA values.
I. Iatrodpetioa In protonated polyfunctional organic molecules, intramolecular hydrogen bonds play an important role to stabilize the ions. The intramolecular hydrogen bond gives stabilization by up to 20 kcal/mol in diamines and amino alcohols.' The intramolecular hydrogen bond is also of biological importance in protonated peptide8 and ionic intermediates in enzyme processes. Gas-phase proton affinities (PA's) of many organic molecules have been measured, and a correlation between PA values and the strengths
of the intramolecular hydrogen bonds has been suggested in ions ~ . ~ studies of protonated di- or of polyfunctional g r o ~ p . These polyether compounds stated that the G H + - -0hydrogen bond plays a central role in PA values and that structural information is needed for rationahhg more clearly the differences of the PA's in a series of organic compounds. Although there are many observed PA's related to the intramolecular hydrogen bond, explicit struchuc analyses have not been made and it is tempting to investigate the PA-~tructurecomlation
0022-3654/92/2096- 10261$03.00/0 Q 1992 American Chemical Society
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