Ferromagnetic versus molecular ordering in C60 charge transfer complexes

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Synthetic Metals 77 (1996) 287-290

Ferromagnetic versus molecular ordering in CeOcharge transfer complexes B. Gotschy a,R. Gompper b, H. Klos a,A. Schilder a,W. Schiitz a, G. Vijlkel ’ a Experimentalphysik II, Universittit Bayreuth, Bayreuth, Germany b Institutfiir Organ&he Chemie, LMIJ, Munich, Germany ’ Experimentelle Physik, Universith’t Leipzig, Leipzig, Germany

Abstract [TDAE]& (TDAE, tetrakis(dimethylamino)ethylene) wasthe first andis sofar the only CeOchargetransfercomplex,which exhibits molecularferromagnetism with a Curietemperature of about16K. Thoughextensivelystudieda convincingaccess to theorigin of the lowtemperaturephaseis still missing.Three mechanisms for the ferromagnetism will be compared:the classicalHeisenbergferromagnet, configurationinteractionandspinpolarization.The roleof molecularorderingandof theJahn-Tellerdistortionof Cmwill bediscussed. Keywords:

Fullerene; Ferromagnetism

1. Introduction The enormousinterest in C, charge transfer (CT) complexes was triggered by the fascinating low-temperature

solid-state properties of some of these systems. One of them is tetrakis( dimethylamino) ethyleneGo (1, [ TDAE] C6,J. The susceptibility of this CeOCT complex exhibits a strong increase below 16 K [ 11. This was claimed to be the sign of a soft ferromagnetic ground state. The Curie temperature T, of about 16 K is the highest transition temperature of a material, which is composed only of first and second row elements. In a recent report Suzuki et al. [2] gave evidence for spontaneous magnetic order in [ TDAE] CeOwith a small remanent magnetization tending towards a classical ferromagnetic ordering of localized moments. In a number of papers Blinc and co-workers ( [ 31 and Refs. therein) interpreted the observation of a saturation magnetization of only 0.1-0.5~.B instead of 2~~ in terms of a correlated spin-glass. Here, we will not focus on the ferromagnetic properties. This article will be devoted to the role of molecular order, and we will discuss our ideas about the origin of the magnetic ordering.

2. Sample presentation

TDAE is known from chemistry as one of the strongest organic donors, and the chemistry of TDAE CT complexes is delicate [ 41. However, tertiary amines like TDAE are quite 0379-6779/96/$15.00

0 1996 Elsevier Science S.A. All rights reserved

attractive as a starting point for the search for new ferromagnetic C6,, CT complexes. One such amine is 1,1’,3,3’-tetramethyl-A *,“-bi( imidazolidine) (2, TMBI) [ 51. An alternative route is electrocrystallization yielding bulk single crystals of ChO radical anion salts. One such salt is [P(C,H,),] &,I. The synthesis of this air-stable salt was first reported by PCnicaud et al. [ 61. 1

2

AX- kMe Me jde Me,

MeNN

p

Me

\

NRMe

he MC

3. Results and discussion

Among the many theories about ferromagnetism in [ TDAE] CeOproposed, only three seem to be relevant: the classical Heisenberg ferromagnet, configuration interaction (CI) also denoted as McConnell mechanism and spin polarization. Their applicability will be discussed concerning two examples of ChO CT complexes: [TDAE] C6,, and

[~BII

Go.

So far all interpretations were based exclusively on the postulation that magnetism is only produced by the &, ani-

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ons. This neglects the fact that we have a two-spin system: the TDAE radical cation and the Cho radical anion, We will discuss the weak links of this postulation later in more detail, but for the moment we will follow this route. The simplest mean field model (classical Heisenberg ferromagnet) predicts for T,: T J.ms+ c 3

a=1 5.8ii

1

1)

where .? is the mean exchange interaction, the spin S is 1I2 in our case and z is the number of next neighbours. From T, andz we can estimated. The unit cell aspublished by Stephens et al. [7] is monoclinic, and the next-nearest neighbour in the b-c plane has the same distance as the nearest neighbour along the a axes. Thus, z = 10 and with T, = 16 K we get j= 3 K, which is a reasonable value for organic ferromagnets. However, if [ TDAE] C& is a classical Heisenberg ferromagnet, it is not reasonable that [TDAE] C6a is the only C6,, ferromagnet discovered in spite of many new synthesized Cc0 compounds. Thus, TDAE is obviously more than just the electron donor, and we must discover the role of the TDAE cations. So far any influence on the magnetism has been denied. Very unphysical, esoteric spin cancellation mechanisms have been postulated, which eliminate the spins of the TDAE radical cations. So we tried again to find the EPR of the TDAE cations. The idea was that the g factor of the free TDAE radical cation is 2.0035 [8] and that of the free C6a radical anion 1.9998 [9]. The EPR lines of both species should be well separated in the Ka band (34 GHz, 1.2 T) . Consistent with all previous reports we found only one EPR line, but it was not symmetric, as had always been reported before; it was a powder EPR line with a nearly axial symmetric gtensor. This is in fact not surprising and simply reflects the symmetry of the unit cell. But the most important finding is the mean electronic g value. It is clearly lower than the g value of the free TDAE radical cation but significantly higher than the value of the free Cc,, radical anion. The reason is that we are in the limit of strong exchange coupling between cation and anion, and the g value is the mean value of both components [ 101, Thus, the TDAE cations are present, and they interact with the C6,, anions. This is clear from Fig. 1. The methyl groups of the TDAE are very close to the C6e. But one should note that, due to the displacement of the TDAE and the Cc0 layer along the b axes, the contact is not as close as this projection onto the a-c plane might suggest. So we can interpret TDAE as a magnetic coupling unit, which suggests that spin polarization is the relevant mechanism, consistent with preliminary calculations by Yamaguchi et al. [ 111. We will come back to this mechanism later and focus now on the role of molecular (or, in the more common notation, merohedral) ordering. It is well k.nown that above 260 K the Cd,, molecules in the solid state rotate very fast and isotropically. One could speak of a liquid crystal. Below 90 K this rotation is frozen out, and

Fig. 1. Projection of the unit cell of [TDAE] C& onto the a-c plane. Indicated are the van der Waals radii of C60and the CH, groups of TDAE.

330 K

-200

-100

0

100

200

frequency [kHz] Fig. 2. 13CNMR spectraof Cm anionsin [P(C,Hs),] &,I temperature.

at 5 K and room

4

Fig. 3. JT distorted C& anion. The arrow indicates the molecular magnetic

the Ceomolecules take characteristic orientations with respect to each other. An electron-rich double bond on one C6e faces a hexagon or an electron-poor pentagon of another C&,, This is called merohedral order. In fact it is the consequence of minimizing the electrostatic repulsion, Fig. 2 shows 13C NMR measurements on a polycrystalline sample of [ P( C6H5)J 2C601.We find a narrow t3C NMR line of the CGo anion at room temperature, which is the clear indication for rotation. Again, here, the rotation is frozen out at low temperatures, resulting in a broad structured NMR spectrum. Since the contact along the c axes is so close, it is reasonable to assume that we also obtain here a merohedral order, i.e., a well-defined orientation of CGoanions relative to each other. The resulting close relation between the properties of the

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et al. /Synthetic

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77 (1996) 287-290

289

Fig. 4. Spin polarization scheme in [TDAE] ChO.

ferromagnetic ground state in [ TDAE] C6e and the merohedral order was recently demonstrated by a paper of Mihailovic etal. [12]. A last important point is the strong electron-phonon coupling in fullerene systems, which manifests itself in the superconducting fullerene intercalates. A consequence here is that the high I,* symmetry of the Cc0 anion is not stable, and it is Jahn-Teller (JT) distorted, leaving a CeOanion with only axial symmetry. The excess charge is concentrated in a belt around the equator [ 131. This gives a net magnetic moment perpendicular to the equator, which is now bound to the molecular lattice (Fig. 3). Thus, the merohedral order supports the formation of ferromagnetically ordered domains, and the spin-glass behaviour can be connected with orientational disorder [ 121. Furthermore, in this CGOsymmetry the spin densities change sign perpendicular to the equator. Negative spin densities are usually required for an effective spin polarization with r wave functions. But the problem is still more complex. Probably the JT distortion is static only at low temperatures (below 140 K [ 141) , and the Cc0anions recover their spherical shape due to pseudo-rotation. In summary we have the molecular order, the spin polarization across the TDAE and the molecular moment due to the JT distortion. All these effects obviously lead at least to a short-range spin-glass-like magnetic order (Fig. 4). The exchange coupling across the long (a) axes is mediated by TDAE, which is the basis of the high T,. Furthermore, spin polarization hampers the antiferromagnetic ordering of the spins of the C,, anions (note that, due to the Pauli exclusion principle, the exchange interaction of two spins in pZ orbitals of neighbouring Cc0 anions must be antiferromagnetic), The picture above is just an illustration of a possible exchange pathway and is based only on hand-waving arguments. For example, the electrostatic repulsion of the excess charge belts is minimal. But this picture also suggests that the c axis is the easy axis of magnetization, whereas Mihailovic et al. [ 1.51 claim that the a or b axis is the easy axis of magnetization. Further measurements are required to clarify this question.

A-...E

t-

+ TDAE'

Go-

1-t

+I

C602-

TDAE2+

Fig. 5. Scheme of the McConnell mechanism in [TDAE]C&,. Table 1 Properties of two &-based CT complexes: E,, E2 in V vs. SCE No.

Refs.

E,

(V)

1

1,4

-0.63

2

5, 17

-0.83

Ez (VI

Go-

&Z-

Solid state properties

-0.75

yes

no

yes

yes

ferromagnet, r,=16K weak paramagnet

A second model which should also be taken into account [ 1 l] is based on predictions by McConnell [ 161. It describes the stabilization of the triplet state of an ion pair D’ ‘A’ relative to the singlet state by configurational mixing with an excited state of the form D*+A”‘-. For the discussion of this mechanism it will be assumed that the three-fold degeneracy of the C,, HOMO is only partially lifted by the JT distortion, resulting in an A and two degenerate E states. The left side of Fig. 5 shows one possible spin configuration. Note that this is not a real high-spin state. The spin orientation is arbitrary, and the system is paramagnetic. A possible excitation is a second virtual CT from TDAE to C6,,. The TDAE dication is probably diamagnetic. The two spins on the Cc0 dianion are in the triplet state. Now this is a real high-spin state due to Hund’s rule, and ferromagnetic spin alignment can propagate through the system. Some remarks should be made about the applicability of this model. It requires that the degeneracy is not completely lifted by the JT distortion. ( 1) In contrast, most theoretical calculations predict a singlet ground state for C6e2-, which is also supported by our measurements on [ TMBI] Cm [ 171. As can be seen from Table 1, the half-wave potential El of TMBI is higher than that of TDAE. We find mainly diamagnetic CT complexes of the form TMB12 + C6e2- . However, the JT splitting will be small

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(of the order of 10 meV [ 131) and a recent report on compounds containing ChO dianions even claims iso-energetic singlet-triplet states [ 181. (2) As can be seen from Table 1, the second oxidation step E2 for TDAE is only 120 meV above the first one. Thus, a McConnell-type mechanism for the ferromagnetic order cannot be ruled out but is probably disfavoured compared to spin polarization.

Acknowledgements

Fruitful and stimulating discussion with M. Schwoerer, and help with the sample preparation by J. Gmeiner and I. Rystau are gratefully acknowledged. This work was supported by Fond der Chemischen Industrie and BASF/BMFT 03M4067-6.

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[4] N. Winbergand J.W. Buchler, Angelo. Cllenl., ht. Ed En& 1 (1962) 406. [51 H.E. Winberg, US Patent No. 3 239 519 (1966). [61 A. PBnicaud, A. Per&-Benitez, R. Gleason, V.E. Munoz and R. Escudero, J. Am. Chem. Sot., 115 (1993) 10 392. PI P.W. Stephens, D. Cox, J.W. Lauher, L. Mihaly, J.B. Wiley, P.M. Allemand, A. Hirsch, K. Holczer, Q. Li, J.D. Thompson and F. Wudl, Nature, 35.5 (1992) 331. [81 K. Kuwata and D.H. Geske, 1. Am. Chem. Sot., 86 (1964) 2101. [9] K. Tanaka, A.A. Zakhidov, K. Yoshizawa, K. Okahara, T. Yamabe, K. Yakushi, K. Kikuchi, S. Suzuki, I. Ikemoto and Y. Achiba, Phys. Lett. A, 164 (1992) 221. [ 101 B. Gotschy, Phys. Rev. B, (1995) in press. [I 11 K. Yamaguchi, S. Hayashi, M. Okumura, M. Nakano and W. Mori, Chem. Phys. Lett., 226 (1994) 372. [12] D. Mihailovic, D. Arcon, P. Venturini, R. Blinc, A. Omerzu and P. Cevc,Science, 268 (1995) 400. [13] For example, see: J. Ihm, Phys. Rev. E, 49 ( 1994) 10 726. 1141 B. Gotschy, M. Keil, H. Klos and I. Rystau, Solid State Commwt., 92 (1994)

References [ 1] P.M. Allemand, KC. Khemani, A. Koch, F. Wudl, K. Holczer, S. Donovan, G. Griiner and J.D. Thompson, Science, 253 (1991) 301. [2] A. Suzuki, T. Suzuki, R.J. Whitehead and Y. Maruyama, Chem. Phys. L&t., 223 (1994) 517. [3] A. Lappas, K. Prassides,K. Vavekis, D. Arcon, R. Blinc, P. Cevc, A. Amato, R. Feyerherm, F.N. Gygax and A. Schenck, Science, 267 (1995) 1799.

935.

[ 151 D. Mihailovic et al., E-MRS 1995 Spring Meet., Stmsbowg, France, 22-26 May 1995. [I61 H.M. McConnell, Proc. Robert A. Welch Fowf. Chem. Rex, 11 (1967) 144. El71 A. Schilder, B. Gotschy, A. Seidl and R. Gompper, Che,n. Ploys.,193 (1995) 321. 1181 For C,*-: P.D.W. Boyd, P. Bhyrappa, P. Paul, J. Stinchcombe, R.D. Bolskar, Y. Sun and CA. Reed, J. Am. Chem. Sot., 117 (1995) 2907.