From benzene to fullerenes through graphite intercalation compounds: A magnetic resonance survey

Synthetic Metals, 55-57 (1993) 3002-3007

3002

FROM

B E N Z E N E TO FULLERENES

THROUGH GRAPHITE

INTERCALATION

COMPOUNDS : A MAGNETIC RESONANCE SURVEY

P.LAUGIN1E, A. MESSAOUDI and J. CONARD. Centre de Recherche sur la Matiere Divisee (CNRS-Universite d'Orleans) 45071 Orleans Cedex 2. France.

ABSTRACT Through a general and critical survey of magnetic ,esonance in Graphite and its alkaliintercalated compounds (including 13C, inserted alkali and conduction electron), useful teachings for doped Fullerenes and conducting polymers are derived. INTRODUCTION We intend in this paper, mainly devoted to a critical review of magnetic resonance in AlkaliGraphite Intercalation Compounds (G.I.C.), to point out some topics of interest both to conducting polymers and new Fullerenes and Fullerides. G.I.C. are highly conductive, highly anisotropic lamellar compounds in which donor or acceptor in~,erted specie,,, are separated by a specific number of carbon planes : the "stage" of the compo~md. We unde,linc that Graphite, a degenerate semi-metal, is thus transformed in a "true" synthetic metal thruugh a shift of the Fermi level away from the band degeneracy region, a subsequent large increase in the basal effective mass, and modifications of the conduction bands which remain nevertheless of dominant rt-character. Extensive information on G.I.C. can be found ill Proceedings and review papers[ 1-7]. A general discussion of the "status" of the conduction electron has been given in 181. ALKALI-NMR 7 Li, 87 Rb and 133Cs NMR Knight shifts (= 1/6-1/3 of the pure alkali one) in stage-I G.I.C. reveal a Fermi level alkali s-orbital populatioa of 5 -15%. On the contrary, no measurable Knight shift could be separated from the (small) chemical shifts in higher stage compounds. So, carbon orbitals admixture in the Fermi level wavefunction reach (very roughly) = 90% in stage-1 and probably overpass 99% in higher stage compounds (191, I 1(11and references therein). Concerning M3C60 alkali-Fullerides, our 133Cs results together with 39K ones recently reported [21] appear quite smilar to 2 nd stage G.I.C., except for the occurrence of 2 types of alkali sites. 0379-6779/93/$6.00

© 1993- Elsevier Sequoia. All rights reserved

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13C-NMR From fomler studies in our group [ 11[ two main fact:~ appeared : - graphene-referred shifts donlinated by the conduction electrons interaction. - heterogeneity of "in-plane" D.O.S. and charge transfe,s in stage >3 G.I.C., strongly

decreasing from the alkali-adjacent C-plane.,, towards the "interior" planes. Later, K. Kume and coworkers, using oriented plates (H.O.P.G.-issued) brought very refined results [12, 13], namely

evidencing in each C-plane two types of C-atoms. However, we disagree with their calculation of the charge transfer distribution among the planes in that the rigid band approximation was uniformly applied to all planes. Instead, in the model we have proposed[S], its use is limited to the little charged "interior" planes for which it is correct. The general scheme of 13C-shifts fronl benzene to G.I.C. is shown in Fig. 1 for each of the principal orientations (a,b) and (c) of tile llXUn l'icld H.. "Graphcne"- a ,,ingle isolated graphite plane - is, as we shall see, a compulsory starting p o e t : we take it as our reference.

Graphene, Benzene, Fullerenes The graphene lines shifts anisotropy r~sults mainly from two interactions [8, 12- I7] :

- H o / / ( c ) : a small diamagnetic shift (= 10 ppm) .,,imilar to the London term in aromatics (interatomic ring currents) and coming to zero in the (a,I)) orLentution. - 1-1o / / ( a , b ) : u very large (16{.I l)pm) orbital parumagnctic >,hift resuhing mainly from the local Van-Vleck interaction /o--+n": and g-~o"{ excitations), quantum-averaged to zero in the (elorientation because of the 6-fold symmetry. This local term will undergo only little changes from benzene to G.I.C., Fulle,enes and Fullerides. This line position is taken as our :ero-shifg reference : it lies 55 ppm flom liquid Benzene or 185 ppm from T.M.S. on the paramagnetic side. The Graphene positions have been in fact deduced from .',pherical or ellipsoTdal Graphite samples

after substracting the very large macro,,coDc fieM botuld to the h@l axial su~,ceptibility. Frozen Benzene position>, are alnlo~,t the ~am¢ its gruphenc ones and qtute close to them also the C(R) ones displayed on Fig. 1 , differing e,,~,cnttally m a lu>.', uf local axial symmetry [ 19, 20]. Thus, G r a p h e n e appears like :m uHimtc aromatic plane, as it should! But this was fully understood only in 1988 [14, 151. Now. comine to G.I.C. and Fullerides Inserting donor species in Graphite rcsuhs in 3 main modifications : charge transfer towards norbitals, shift of the Fermi level resuhing ill large Fermi D.O.S., hybridization (more or less) of ~orbitals with intercalate and with o or othvtr Carbon orbitals. Those effects are now examined. Orbital shifts in G.I.C. : both principal orbital .shifts a,'e differently affected : -

H . / / ( e l : a 10-30 ppm dian,a,~nefic shift (paradoxical '?) for carbons in "adjacent" planes

in alkali G.I.C., maybe related to increasing ettectlve mass and decreasing radius of current loops. - H o / / ( a , b ) : charge transfer iutu n-urbitals cause their expansion. The orbital paramagnetic local field is thus reduced, leading to a small diamagnetic shift estimated by R. Saito et al [161.

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Fig. h General scheme for 13C-shifts. The C60 spectrum is taken from 120]. The highest C60 peak due to partially rotating carbons, serves as a mark for the isotropic shift.

3005 r

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Fig. 2. 130 isotropic shifts and the ,',lgn of the contact interaction (data from 112, 13, 16]). The abscissa are proportional to the local Fermi D O . S . Broken line : as-reported by the author~, (close to tile linear regression line of the whole set) Full line : out proposed interpretation Fig. 3. E.P.R. relaxation time T2 versu:, alkah atomic number Z o 2 nd stage G.I.C. + M3C(~0 Metallic shifts in G.I.C.: we mean those shift.s proportionnal to the local Fermi D.O.S. : their isotropic part or "contact" term arise.,, from a l=crmi electrons density on the Carbon nucleus while their anisotropic part arises from tile distant dipohtr interaction wqth those electrons. - Dipolar tern1 : this traceless interaction leads to large opposite shifts tot the (a,b) and (c)

c o i n p o n e n t s in such a way that tile ~,hift-ani',otropy is reduced and even compounds[11-13].Those

reversed in 1 st stage

shifts are very ~,ltlal[ ill acceptor G.I.C. (small Fermi D.O.S.). The

anisotropic part of the hyperfine field ( c o n t a c t + d t p o l a r / o f ,t re-orbital is evahlated to - 1.5.105 Gauss (or more if a strong electron-phonon enhancetncnt i~, taken 111lo account). - contact teFm : tile hne~. are then slighfl 5, i,,otrop~cally ~,hifled through the contact interaction, thus finally cotnillg to the puMllUil whcfc [hv.' 5 actmflly lic! ITof a tong time this last interaction was supposed to be indirect (diamagnetic-hkc), ~-urbitals ha,,ing no density on the C- nucleus. [12,131. W e believe it on the contrary to be paramagnetic tot direct) and this point is of interest for Fullerenes. It is shown on Fig. 2 (on which me reported authors proper data) that, when excepting 2 "anomalous" points, the left points fit with much higher accuracy a straight line of opposite slope, thus revealing a paramagnetic-hke interaction/predominating direct teml). From which we deduce the isotropic part of the hyperfine field to be := + 800(/Gau.',s (or more). If we accept the negative core polarization contribution estimatect in [ 13 J, then the direct one would reach = 18000 Gauss (or more). In fact, this resuh had been foreca.,,ted by D,. Saito et al in their attempt to calculate 13C- shifts in K-G.I.C. [16,17] : a small (5-~ hybridizam,m was invoked originating a direct contact interaction (also mainly responsible tot-the 13C doublct,,~. Th:,, ~,ecmcd tu first contradictictory to the as- reported e x p e r i m e n t s , we clainl, on the contrary, tho,,c one,', to bring a strong support to Saito's theory. There are some reasons for tile stage- I point not to lie on the same line as higher stages ones because of incomplete charge tran:,fer (relaxatton rate po,,,,ibly modified by the distant dipolar

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interaction with s- alkali orbitals). The ca.,,c or the 3 rd plane in stage 6 G.I.C. is more intricate. A possible explanation in terms of orbital effects (not traceless!) ha.,, been suggested in [81. and in Fullerenes and Fulleridcs : the cs-rc hybridization ha.', been supposed to take a more prominant part in Fulle,ene.s because of C-plane.,, curwttur¢. So, it was already present in G.I.C.! Isotropic shifts in C60 effectively reveal an increased direct contact interaction (Fig. 1). M3C60 results presently at disposal (M=K, Rb [21, 221 and ou, preliminary Cs ones), obviously subject to the above analysis, also conclude to an inc,'eased direct contact tema. E.P.R.

We shall take interest below only in linewidths versus alkali atomic number Z. The most striking fact is tile near Z -4 dependence of the relaxation time T2 (or reciprocal linewidth) ibr a given stage K-Rb-Cs series (Fig. 3 ). It proves the linewidth to be dominated by a spin-orbit coupling with the alkali. As it is extremely effective with heavy alkali, a very small admixture of alkali states is sufficient to account for the observed broadening[2 (p.514), 8, 9]. Then, our EPR studies on a M3C6o series (M= K, Rb, Cs) brought us immediately a very nice result : roughly same linewidth values ancl Z-dependence as in second stage alkali-G.1.C. (Fig.3)! K. Sugihara linewidth's theory [181 seems> uncompleted since the part taken by the alkali has been limited to stage 1. Briefly, f,'~cts :ire : the higher the stage and the more splitted the ~-bands, the more efficient the spin-orbit interaction with the alkali. Why '? (a good theoritical challenge!). A similar remark has been made in I,";[ a~, concern the 13C-doublets : the part taken by the alkali in the differentiation between both types oi C-atom:-, was restricted to the "adjacent" planes in [13, 14]. CONCLUSIONS

From this rapid survey some

main

fact~, t u n i c into view :

- Graphite and G.I.C. are good "moclel .',y.,,tcms" for Eullerides and conducting polymers. - High adaptability of the carbon boncl~, tu "new" situations again confillned (if ever in doubt!). - High selectivity of charge transfer tm~ ards C-planes and/or inequivalent C-sites. - Be careful! Rigid band approximation only wdid for very small charge transfers. - Notwithstanding their general cubic .',ymmetry, Fullerenes and Fullerides when observed with local probes are much more like Benzene and donor G.I.C.respectively. More particularly, the similarity between M3C60 and stage-2 G.I.C. b, complete fiom alkali NMR and EPR, and to a lesser extent 13C-NMR. - t s - x hybridization, originating a parai'nagnctic contact hyperfine field in Fullerenes and

Fullerides is al,eady present tit a smaller c,:telu in G.I.C.. Some topics which came alorig the di~cu.,,sions would de.,,erve improved theoritical research : - Graphite and G.I.C. are typical .,,y~,tem~, in which the local paramagnetic orbital currents, producing large local fields give little cunwibution to the susceptibility while the contrary is true for the large orbital "ring" currents giving xb,e to the high Graphite axial susceptibility (in other words, 1st order perturbation te,'m dominated hy the 2 nd order one) ; this is related to inverse r-dependences.

3007

- However, a unified tveutmc~at of 13C tnt)~tat >,hitth -Ior in>,iaJ]ctc, the gauge-i~lvariant Fukt~yama formalisnl applied with pzrtrtial .,,tlcces.,, to the ~,t)le pure Grztphite in [17] - would be welcome for G.I.C. and (intercalated) Fullerenes. Notice that the occt~rrence of a single line instead of a doublet in the (a,b) orientation of pure Grztphite h',t.,, never been z~ct.'ounted fo, ,n a~y theory. - Increasing effective mass may give rise to increasing diamagnettc shifts! - The range of the interaction with alkali ~uclei seems to exceed the "adjacent" planes to which it

had been supposed to be rest,icted. - A completed theow fo," EPR li~aewidths remai~s quite desirable. Summing Lip : difficult, but fasci~aati~ag! REFERENCES 1 I.S.G.I.C.-1 : M a t . Science and En~,ineering. 31 (1977).

I.S.G.I.C.-2 - 5 : Svnth.Met., 3 (1981) ; 7 (1983) ; 23 (1988) ; 34 (1989). I.S.I.C.-6 : Mat. Science Forum, 91-93 (1992). 2 Layered Mat. and Intercalates. Phv.sica. 99 B+C (1980). 3 Physics of Int. Compounds, Springer series i~1 Solid State Science, 38 (1981). 4 M.S. and G. Dresselhaus : Int. Cpd~,. orGraphite, Adv. in Phys., 30 (1981) 139-326. 5 Graphite Int. Compounds, Svnlh. ~Vlet., 12 (1985). 6 H. Estrade-Szwarckopf, Helvetica l:'h>,sica .'~cta. 58 (1985) 139 7 Chem. Physics of hat. Compotmd.,,, NATO ASI Serie.,,, series B, 172 (1987). 8 P. Lat.lginie, These d'Et:lt. Ulai,,,ei.,,it6 Ptlris-Sud, Or.,,:ly,Fr:mce, 1988. 9 P. Lauginie, Synth. Met.. 23 (1988t 311. 10 P. Lauginie, H. Estrade-Szwztt'l.'kt.)l]l ~tllclJ. Cot~tud, Mat. Sciellce Forum, 91-93 (1992) 545. 11 J. Conard, M. Gutlcrrez-Le Brutl, I°, l.ztt~gmle, H.E~trade-Szwarckopl and G. Hermann, Svnth. Met., 2 (1980) 227. 12 K. Kume, K. Nomur:t, Y. llii+o.\,:tmzt, Y. Mzttl~x~,'a,lq. SttCtl/ztt:-,Ll:ltld S. Wanullqa,Swlth. Met., 12(1985) 307. 13 Y. Mavtiwa, K. Kume, H. Suemat.-,u zttad S. Talat.lma, J. Phv.,,. Soc. Japan, 54 (1985) 666. 14 Y. Hiroyama zmd K. Kume Solid State comm.. 65 (1988) 617. 15 P. Lauginie, H. Estrade-szwarckopf, B. Rous.seau iliad J. Conard, C.R. Acad. Sci. Paris, 307, Ser. II (1988) 1693. 16 R. Saito, M. Tsukada, K. Kobayashi lind H. Kamimura, Phys. Rev.B. 35 (1987) 2963. 17 K. Kobayashi and M. Tstlkad~l, l)h\'s, l~,e\,. B. 38 (1988) 8566. 18 K. Sugihara, J. Phys.Soc. Jz~p.. 53 t1984) 3t)3. 19 C.S. Yannolli, R.D. Johl~.,,on, G. Met.let, D.N. Bethulle ;rod J.R. Salem, J. Phys. Chem.. 95 (1991) 9. 20 R. Tycko, R.C. HaddoN, G. DaI-Jktgl~. S.tl. Ghlrtlm, D.C. Dougl~tss and A.M. Mtljsce, J~ Chem. Phys. 95 (1991) 518. 21 K. Holczer, H. Alloul and D. KleiJl, lgHX,ztte commulllcatt~ll. 22 A.A. Zaha[~. thrse, Uni,,'elsit6 de N,lol~tlgellier, Frtmce, 1991.