Heterocomplex-based graphite lamellar compounds: C22CuAl2Cl8.5 and C10Cd0.2AlCl3.7: Intercalation pathway, structure and transport

Carbon Vol. 34, No. 1, pp. lOl-107,1996 Copyright 0 1995 Elsevier Smnce Ltd Printed in Great Britain. All rights reserved 0008-6223/96 $15.00 + 0.00

Pergamon 000th6223(95)00141-7

HETEROCOMPLEX-BASED GRAPHITE LAMELLAR COMPOUNDS: C,2CuA1,C18,, and CloCdo,2A1C1,.,: INTERCALATION PATHWAY, STRUCTURE AND TRANSPORT E. MCRAE, V. POLO, R. VANGELISTI and M. LELAUBAIN Universitt Henri Poincart, Nancy I-Laboratoire de Chimie du Solide Mineral, U.R.A. C.N.R.S. 158, Service de Chimie Mintrale Appliqie, B.P. 239, 54506 Vandoeuvre les Nancy Ctdex, France (Received

17 November

1994; accepted in revised form 11 July 1995)

Abstract-Attempts to intercalate the chloroaluminate complexes CuAl,Cl, and CdAl,CI, into graphite have resulted in the synthesis of two new first stage compounds, C,,CuAl,Cl,,, and C,,Cd,.,AlCl,.,. The former possesses an oblique in-plane unit cell, the latter a rectangular cell, commensurate with that of the host lattice. The electrical resistivity has been studied within (p.) and perpendicular (p,) to the basal planes from 295 to 4.2 K. Both dJdT and d,=/dT are positive (metallic) and the anisotropy p,/p, lies between lo5 and lo6 over the full temperature range. The data are analyzed and discussed in terms of the structure and current transport theories. Key Words-Graphite,

intercalation,

electrical

conductivity.

1. INTRODUCTION Extensive

work

carried

out

over

the

past

years

has

led to the development of a wide range of novel graphite intercalation compounds (GICs)[ l-43. The unusual transport properties, some of which have been reviewed in Ref. 4, include reports of superconductivity in the liquid helium range for alkali metal rich binary phases [ 51, two dimensional (2D) weak localization (quantum interference effects)[ 61 and ratios of in-plane to transplanar conductivities in excess of lo6 for several GICs[ 71. While certain specific features of hole (electron) transport have been explained in acceptor (donor) type GICs no consensus exists at present as to the conduction process(es) in the c-direction perpendicular to the basal planes. In the present work, we have studied both the in-plane and c-axis electrical resistivities, pa and pc, respectively, in the temperature range T= 4.2-295 K for two new, first stage metal chloroaluminate M =Cu and Cd) GICs based on (M,Al,Cl,, Madagascar single crystal graphite (SCG) and highly oriented pyrolytic graphite (HOPG)[ S]. Past transport studies on related materials by our group have dealt with a number of aluminum chloride-containing GICs including stage l-4 binary compounds[9] as well as bi-intercalation compounds with alternating layers of AlCl, and one other di- or trichloride [ 10-121. Details concerning the synthesis conditions and structural characterization for the M = Cu case have been presented elsewhere[8,13-151. It is of interest to this work to note that magnetic studies on the copper chloroaluminate GICs have revealed the existence of a rather unique (to GICs) finite chain distribution of spin l/2 Cu2+ ions below about 30 K[ 161. Finally, we have also investigated the

materials for which M =Pd; however, these results will be presented elsewhere [ 15,171. In this paper, after a presentation of experimental details and structural and electrical characterization, our discussion will center on three aspects of these new members of the GIC family: (i) correlation (or lack thereof) between the structure and chemistry of the initial complexes and the final GIC; (ii) data fits and their analyses in the two directions; (iii) the relevance on transport properties of the HOPG or single crystal nature of the initial graphite and a discussion on the mechanisms implicated in the conduction processes.

2. EXPERIMENTAL

In spite of their chemical similarities, the chloroaluminates of copper and cadmium MAl,Cl, (M = Cu or Cd) react completely differently with graphite [S]. The vapour phase reactions were carried out at 300°C for a reaction time of 3 days in both cases. While the reaction with the copper heterocomplex leads to a first-stage compound approximately formulated C,,CuAl,Cl,,,, that based on the cadmium complex systematically leads to a stageone ternary compound with a low transition metal content, C1r,Cd0.2AlCl,,,, or equivalently, C2,(CdCl,),.,(AlCl,.,),. The graphite used was generally Union Carbide ZYB HOPG and in a few cases, Madagascar SCG. The use of the latter allows both in-plane structural studies and comparison of transport properties to those of GICs based on HOPG, as discussed below. All transport samples were in the form of 4 mm diameter discs, a few 1Oths of a mm thick. (001) X-ray diffraction analyses were done to evaluate stage 101

102

E.

MCRAE

fidelity and to determine thermal expansion as a function of temperature, and (hk0) and (hkl) studies to characterize in-plane (two-dimensional) and threedimensional order. The procedures for the resistivity studies were the following. After synthesis, samples were transferred under inert atmosphere into flat bottomed Pyrex tubes. The in-plane resistivity measurements were carried out inductively decreasing T from 295 to 4.2 K over a 7-10 hour period and temperature was determined using a calibrated thermocouple. Summarizing the c-axis technique, a given sample was transferred under helium gas to a holder in which it was sandwiched between two coaxial platinum contacts, which exerted light pressure on each of the two faces. The outer furnished the d.c. measuring current (l-5 mA), the inner being the voltage probe. Measurements here also were made upon reducing the temperature slowly. After overnight reheating to ambient temperature the samples were again transferred to their flat-bottomed Pyrex tubes, 295 K basal plane resistivity remeasured, and in some cases (001) studies done again. These latter two tests confirmed that the transfers had not resulted in detectable sample damage. A comment might be in order as to whether the measured p,(T) is intrinsic or not. We strongly believe that what we observe is, indeed, at least an intrinsically minimum value of c-axis resistivity. This is based on the following observations. First, in all the available GIC literature, while the ambient temperature basal plane resistivity decreases upon intercalation by an intercalate-dependent factor of 2-15, the total range of c-axis values at room temperature covers roughly five orders of magnitude[7,18]. Sample-to-sample variations in absolute values of pc, for a given stage and intercalate, differ from each other by a factor of, at most, three[7,9,18]. We thus conclude that, particularly in the most anisotropic

et al.

GICs, the contribution of basal plane resistivity to overall measured c-axis resistivity must be negligibly small. That p,(T) does not contain any essential contribution from p,(T) is further brought out by the fact that the former is strongly stage-dependent and the latter almost stage-independent. 3. RESULTS As regards structural definition, these two GICs are characterized by interplanar distances di of 948 and 955 pm for M=Cu and Cd, respectively, compatible with the intercalate sequence Cl-M-Cl within the gallery. In Fig. 1 we have shown the relative variation of di (Adi/di295K) with 7’ for the copper chloroaluminate GIC as well as for the related first stage binaries containing AlCl, and CuCl,[ S]. The coefficients of thermal expansion, (AdilAT)(&s ~))‘a evaluated over the linear portions shown in Fig. 1 are all between 32 x lO-‘j and 39 x 10m6 K-r, values typically found for di- and trichloride-intercalated graphite[ 191. Figure 2 presents the (hk0) electron diffraction patterns for the two cases. In CzzCuA12Clss, the intercalate is organized in the plane as an oblique unit cell: ai = 2579 pm, bi = 662 pm, ‘/ = 122.64”, somewhat similar to that of the (a,b) planes of the free chloroaluminate[ 14,151. One surprising and interesting point of the compound C,,Cdo~,AlC13,, is its two dimensional organization involving a rectangular unit cell commensurate with the host graphitic lattice: ai = 6a, + 36,, b, = 5b,, (ai = 1277 pm, b, = 1228 pm). In the c-direction, the intercalate and graphene layers are arranged in the sequences AclBctAcl and ActBPActB _. for C,,CuAl,Cl,,, and C,,Cd,,,AlCl,,,, respectively. In both compounds, therefore, the intercalate layers possess (different) three dimensional order and the same .&&A graphene sequence found in almost all chloride GICs.

0

-0.002

. .* .

Temperature (K)

Fig. 1. Relative variation in interplanar distance d, vs temperature for first-stage GICs based on intercalation of CuAl,Cl,, CuCl, and AlCl, (latter two, Ref. 8).

Heterocomplex-based

graphite lamellar compounds

103

In Figs 3-5 we compare results for the two families. As concerns basal plane resistivity, sampleto-sample variations are comparable to what is observed in all families studied to date [7, and refs therein]. Figure 4 shows that all c-axis behavior is ‘metallic’, in the sense that dp,/dT>O over the full T range and finite values of p,(T+0 K) are observed. This again is a commonly observed feature in low stage compounds [4,7]. The most highly conducting compound in this direction is the SCG-based sample. The variation of anisotropy is presented in Fig. 5. All values are greater than 10’ over the full T range. Because the c-axis resistivity is generally less Tsensitive than the basal plane resistivity, in most cases there is a small rise in A as T decreases.

..

4. DISCUSSION

4.1 Chemical and structural aspects Among the outstanding structural results are the following.

(4 l * 0. .

.

.

In spite of the similar chemical natures of the starting heterocomplexes the final products are significantly different. The heterocomplex nature of the intercalate does not seem to have significant influence on the interplanar distances: as for almost all di-, tri- and pentachloride GICs, di is around 950 pm. The c-axis parameter is always equal to 2di. Stacking of graphene layers does not depend on the state of organization within the intercalate layer; in almost all chloride-containing GICs, the chlorine seems to impose the A/B stacking of the graphene.

I .

.

..e..# ... 7.......0 . .

.

.

.

.

.

.

.

.

.

.

.

.

.

I

. .

. I

. .

.

.

.

The first point seems particularly intriguing and be linked to the intercalation mechanism itself. As indicated elsewhere[8,14,15], synthesis of the Cu-containing GICs is an isostage process beginning with the pre-intercalation of AlCl,. Subsequently, the copper dichloride is co-intercalated by chemical transport via the gaseous complex CuAl,Cls leading to formation of the ternary (AlCl,, yCuCl,)-GIC. While the structure could not be determined, it appears to bear no relation to those of either the free or intercalated chlorides. By increasing the reaction time, this compound totally disappears, leading to first-stage C2,CuAl,Cl,,,. A small excess of chlorine is thus observed, as in other chloride GICs, usually explained by the fact that intercalation initially requires some excess chlorine in the reactor, acting either as a catalyst or as a charge transfer initiator[21]. As in the case of copper chloroaluminate, the intercalation of cadmium chloroaluminate involves formation of an intermediary compound (C,,Cd,,,AlCl,~,) with a low, divalent metal content; however, increasing the reaction time leads to mixed formation of a layer compound C,,6(CdC1,.,)(A1Cl,,3)0.1 very similar to that observed previously [ 221 using chemical transport techniques (C,,,CdCl,(AlCl,),,,). In no case was it possible to

must

I @I Fig. 2. (hk0) diffraction patterns; (a) C,,CuAI,Cl,., and (b) C,,,Cd,,,AlCl,,,. The large unit cell is that of graphite in both cases; the small one is that of the intercalate.

Let us now examine the results of the transport studies in the two directions. In all, four C,,CuAl,Cl,,, and two C1&do,,AICl,~, samples were studied. The room temperature and liquid helium temperature values of pa, pc and anisotropy A= p,/p,, have been summarized in Table 1 along with results concerning the related AlCl,[9], CuCl,[ lo] and CdCl,[20] binary GICs. The table shows, perhaps somewhat surprisingly, that the HOPG-based copper heterocomplex-containing GICs have residual resistivities in both directions lower than those of the AlCl, binary GICs.

E. MCRAE et al.

104 Table 1. In-plane

(p,) and c-axis (p,) resistivities and anisotropy (A=p,/p,) samples of this study and the related binary GICs” p.Wcm)

Intercalate CuAl,Cl,

(CdCl,),AICl, AlCl, CuCl, CdCl,

Reference

295 K

PT41 PT42 PT44 PT47 PT43 PT49 Ref. 9 Ref. 10 Ref. 20

3.3 3.65 3.3 7 4.6 4.5 3.X-4.6 5.4 78

“Reference numbers (Madagascar SCG).

PT41

to PT49

4.2 K 0.78 0.82 0.73 1.5 0.85 1.37 1.22-1.72 2.0 16

I

I

I I

I

200 100 150 Temperature (K)

295 K

4.2 K

1.37 1.53 0.99 0.7 1.59 1.97 1.58-2.58 1.21 .072

0.33 0.40 0.20 0.23 0.55 0.74 0.44&0.98 0.3 0.02

as in Figs 3-5 and Table 1. All samples

I

250

I

300

Fig. 3. Basal plane resistivity p,(T) for samples of (a) C&uAl,Cl,,, and (b) C,,Cd,,,AIC13,. Sample references as in Table 1. From top to bottom (a) PT47, PT42, PT44, PT41 (the lowest curve has been downshifted by 0.5 @cm to avoid superposition); (b) PT49, PT43.

obtain a CdAl,Cl,-type GIC. The intermediary compound mentioned above presents the particularity of possessing an ordered structure involving a rectangular unit cell, (ui = 1277 pm, bi = 1228 pm) commensurate with the host graphite lattice, but unrelated to the free halide structures. Because of this, the intercalated layer cannot be assimilated to a mixed layer of the (CdCl,,,),,,(AlCl,.,)-type with separate di- and tri-chloride phases, but rather to a solid solution with very limited substitution of Al atoms by those of Cd. The same phenomenon was noted in the ternary compounds that result from the reaction of cobalt chloroaluminate for which the intercalated layer adopts an incommensurate hexagonal structure (a= 599.7 pm, y(a,,ai) = 30”)[ 81. These new ternary phases, the structures of which seem to depend on the substituted divalent metal, are under study.

295 K for

105 x PC/P,

I

50

and

u,(.Qcm)

50

0

0

at T=4.2

I

295 K

4.2 K

4.1 4.2 3.0 1.0 3.5 4.4 3.6-6.5 2.2 0.009

4.2 4.9 2.7 1.5 6.5 5.4 2.6-7.8 1.5 0.013

HOPG

except

PT47

L

6

2Kl 100 150 Temperature (K)

t

2so

301)

0 0

50

100 150 Temperature

200 (K)

250

300

Fig. 4. c-Axis resistivity p,(T) for (a) C,,CuAl,Cls., (top to bottom: PT42, PT41, PT44, PT47) and (b) C,,Cd,,AICl,, (PT49, PT43).

The second and third points taken in the light of all work on chloride-containing compounds shows the important role played by the chlorine. Indeed, as mentioned above, the presence of the chlorine-metalchlorine sandwich always seems to favor (perhaps even to impose) A/B stacking on either side of the intercalate layer. Whether this is due to what is required by the presence of the chlorine atoms themselves or to strong graphene-graphene coupling independent of the nature of the intercalate layer is uncertain. However, the presence of such stacking sequences in first stage chloride GICs in which the intercalate layers are ordered two- or threedimensionally suggest that stacking is dominated by steric effects due to the presence of the chlorine: a compact chlorine atom arrangement appears better accommodated by A/B than by A/A graphene stacking.

Heterocomplex-based

,

L

0

SO

1

100 150 Temperature

,

I

200

250

105

graphite lamellar compounds

300

(K)

Fig. 5. Anisotropy A=p,(T)/p,(T) for (a) C,,CuAl,Cl,., (PTPT42, PT41, PT44, PT47) and (b) C,,Cd,,,AlCl,,, (PT49, PT43). 4.2 Datajits In an attempt to understand the electrical behavior of these compounds, we carried out fits to the p(T) data in both directions using second order polynomials: p(T)=p,+aT+bT2. The fit parameters are given in Table 2. For the SCG-based sample (PT-47), roughly twice as resistive as the HOPG-based samples over most of the T range for pa, all three fit parameters are also multiplied by a factor of roughly two. This would imply that the relatiue contributions of each term are HOPG- and SCG-independent. As seen in Table 2, for M =Cd, the linear term is more significant than for the Cu-containing GICs. Based on the fit parameters of Table 2, one can determine the relative contribution of each term to the overall resistivity. In both directions, curves showing the relative contributions as a function of temperature are all very similar in spite of the orders of magnitude difference between absolute values of basal plane and c-axis resistivities, a fact that certainly suggests that the same mechanisms may be operative in both directions. The linear term contributes to a greater extent for M = Cd than for Cu as indicated

above and its relative contribution for both compounds is greater in the c-direction than in the basal plane. The quadratic term dominates at room temperature, to a greater extent in pa than in pc. Before discussing the meaning of the fit parameters, we comment on the magnetic effects observed in C,,CuAl,Cl,,,[ 161. The intercalate layers in several GICs are known to possess either intra- or interlayer magnetic ordering and in several cases transport anomalies are associated with the ordering temperatures. In particular, the CoCl, binary GICs have been quite extensively studied[23-251 and, for first stage compounds, a sharp rise in pE is known to occur below the temperature at which successive CoCl, layers start to couple antiferromagnetically. Rancourt et al.[26] have also made magnetic studies on the binary CuCl, GICs and shown the existence of a maximum around 70 K in the curves of susceptibility versus temperature similar to that associated with the magnetic effects in CuAl,Cls-intercalated graphite [ 161. The data were interpreted in terms of finite chains of antiferromagnetically coupled S = l/2 ions. As for C,,CuAl,Cl,., of the present study, in the CuCl, GICs no anomaly in transport properties has been observed either in the relevant T range [ 10,271. We thus conclude that the delocalized charge carriers are not sensitive to this type of in-plane quasi-one-dimensional magnetic coupling as opposed to the interlayer coupling in the CoCl, GICs. Returning to the quadratic nature of the p(T) relationship, it is of interest to note that in highly anisotropic systems, the interaction between carriers and phonons can give rise not only to the ‘classical’ T-linear term but also to nonlinear contributions. In the case of GICs, this was explicitly demonstrated for p,(T) in the work of Pietronero and Strassler on AsF,-GICs[28], in which the hole interaction with both graphene and intercalate phonons was treated, and by Kamimura et aI.[29] who put forward interpocket scattering as yielding a nonlinear-T contribution to the overall resistivity. 4.3

Transport theory As we have pointed out above, the single crystal or polycrystalline nature of the host graphite does not seem to play a critical role in the experimentally determined transport coefficients and their variation with temperature. Intergrain boundaries and stacking faults do play a major role in explaining the different

Table 2. Parameters of fits to p =pO + aT+ bTZ for the six samples studied here

Intercalate CuAl,Cl,

(CdCl,),AlCl,

Reference

P0a

103 a

lo5 b

Pot

103 a

lo5 b

PT41 PT42 PT44 PT47 PT43 PT49

0.71 0.79 0.72 1.49 0.85 1.34

2.29 1.86 1.0 2.95 4.15 3.07

2.23 2.55 2.52 5.85 2.91 2.57

0.30 0.37 0.18 0.22 0.53 0.70

1.59 1.71 1.25 0.69 1.26 2.32

0.68 0.80 0.51 0.33 0.79 0.67

106

E. MCRAE

temperature dependences of c-axis resistivity in the two virgin materials[30-331; however, as has been pointed out [ 341, one of the striking effects of intercalation is that the sign of dp,/dT in the resulting GIC is not a function of the nature of the starting graphite. This is undoubtedly a consequence of the fact that intercalation into SCG never leads to a final truly single crystal sample. In particular, c-axis coherence lengths are much smaller than those in the unintercalated graphite. The particular copper chloroaluminate-containing SCG-based GIC of this study is a true single crystal from the diffraction viewpoint within the plane. The basal plane structure certainly bears long range order (LRO) with a correlation length of the order of 1 pm, very large indeed for a GIC, if not the largest observed to date. It is, furthermore, of the same order as the in-plane domain size in virgin HOPG. Comprehension of transport phenomena based on fitting experimental data to polynomial functions, as opposed to a mechanism-specific model, obviously merits precaution. However, when more specific models are used, parameters based on other types of studies are called for and in the present case such studies have not yet been done. Aside from the magnetic studies[ 161, the present work is the first to examine any physical property Such a simple fitting procedure, however, has been used in numerous works both for in-plane studies [27,35-381 and c-axis work[ 35,381. In the case of GICs, while band conduction is generally accepted as being the in-plane process, no general consensus exists as concerns the conduction mechanisms in the c-direction. Indeed, the Japanese school rejects band conduction as being present in a number of ternary donor compounds[39,40] and more generally in the high p,[41] GICs because the Ioffe Regel criterion[42] is apparently not satisfied, based on free electron approaches. We return to this point below. On the other hand, studies that combine both transport and quantum oscillation results lead to exactly the opposite conclusion and show that quasitwo-dimensional undulated Fermi surfaces can indeed lead to anisotropy values of the order of 103-lo5 for SbCl, 1431 and CdCl, [ 351 intercalated graphite, values of the order of those observed here. These authors showed that in the acceptor compounds they examined the conductivity anisotropy could be expressed as: P, pa

3n k:, 21, (k$,-&)3’2

where kF1 and k,, are the Fermi vectors corresponding to the extreme cross sections of the slightly undulating but essentially cylindrical Fermi surface. The effective mass of carriers on the cylindrical portion is infinitely great so that they do not contribute to conduction in the c-direction. The small zcomponent on the undulating regions would then explain the small c-axis conduction. Furthermore, it is obvious that the second term dominates the aniso-

et al.

tropy expression, I, playing only a minor role. Finally, analysis of work carried out on transport under pressure in the case of SbCl, GICs has also led to the conclusion, after comparison of both band and that band conduction is hopping approaches, plausible [ 441. As a final comment, the meaning of the Ioffee Regel criterion and the possible need for its renormalization in layered anisotropic solids has recently been discussed at length by Xie [45]. This author expressed the tight binding energy spectrum of a layered system by E= -cos

k,-cos

k,-ices

k,

in which i represents the hopping integral of carriers along the c-direction, 1 ~
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