The electric resistivity of compacts of graphite intercalation compounds with antimony pentafluoride and with antimony pentachloride

Carbon Vol. 17. pp. 195~2W 0 Pergamon Press Ltd.. 1979. Printed in Great Britain

THE ELECTRIC RESISTIVITY OF COMPACTS OF GRAPHITE INTERCALATION COMPOUNDS W ITH ANTIMONY PENTAFLUORIDE AND W ITH ANTIMONY PENTACHLORIDE L. STREWINGER,H. P. BoEHM,tR. SCHL&L and R. PENTENRIEDER Institut fiir anorganischeChemieder Universitat Miinchen, MeiserstraEe1, D-8000Miinchen 2, Germany (Received 23 January 1979)

Abstract-The electric resistivities were measured for textured compacts of the intercalation compounds of graphitewith SbF, and with SbC&.Starting materialswere coarseflakes of natural graphiteand graphitefoils (0.35 and 2 mm thick). First stage compoundsand mixtures of first and second stage compoundswere studied. The resistivities of the compacts from flakes were strongly pressuredependent,maximum conductivity was not yet reachedat 600bar. The resistivities of preparationsfrom graphite foils were much less pressuredependent.The resistivity of the foils decreasedto 36 x lo-’ am, that is 3-4% of the resistivity of the parent foils. In general,the intercalationcompoundswith SbFsshowedsomewhathigher resistivitiesthan the SbC&compounds.This seemsto be causedby formation of an insulating layer on the surface of the particles due to partial fluorination of carbon atoms with SbFs. C-F bonds were detectedby IR spectroscopyand by XPS. The first stage SbFs compound is formed already at room temperature,contrary to literature reports, provided sufficienttime is given.

1. INTRODUCTION

The electric conductivity of graphite in the direction of the layer planes increasesdrastically upon formation of charge-transfer intercalation compounds[ I]. Interest in graphite intercalation compounds increased remarkably after Vogel reported that he found a conductivity surpassing that of copper using a composite wire with a polygranular core of an intercalation compound of graphite with antimony pentafluoride121.Stage and composition of this compound remained unknown. The high conductivity observedby Vogel with a polygranular material could not be confirmed by Thompson et a/.[31 and by Fuzellier et a1.[4] who used highly oriented pyrographite (HOPG) as starting material. Both groups, who measured the conductivity by the usual Ccontact d.c. bridge method, found resistivities of the order of p=s... 8 x lo-’ Rm for the graphite compound with SbFs (pcu = 1.7x lo-’ 0rn = 1.7@cm). The first and second stages had higher conductivities than the higher stages.The conductivities of the SbCls compoundswere comparable to those of the compounds with SbFs[4]. Both groups complained of difficulties with the electric contacts to the very corrosive samples. Zeller et a1.[5] pointed out a little later, however, that the d.c. bridge technique is not suitable for measurements with materials possessingsuch high an anisotropy of conductivity as is observed with these intercalation compounds(u,Ju~ = ca. 106)becausethe current density is not uniform over the entire cross-sectionof the sample. Radio frequency methods should be used instead. Using this technique,the authors found a conductivity of 6.2 x lO’(Rm)-’ (p = 1.6x lo-‘Qm), equalling even that of silver, for AsF5-graphite[6]. The second and third stages had the highest conductivities[7,8]. There was a wide scatter in the individual measurements,however. tTo whom requestsfor reprints should be addressed.

The use of HOPG for practical purposes seemsto be out of question in view of its price, its limited size, and of its rigidity which does not allow winding of coils. W e studied therefore the conductivity of polygranular intercalation compounds with SbFs and with SbC15.Since conductivity is high only in the direction parallel to the layer planes, a strong preferred orientation of the crystals of the graphite compoundswas desired. Large crystals would reduce the contact resistances. Flakes of natural graphite were used as starting material, and compactswere obtained by pressingtextured samplesof more or less well-alignedflakes. However, foils made by compressingexfoliated graphite (prepared from graphite hydrogen sulfate or nitrate) seemed to be still better suited becausethe individual crystals form a dense network with the layer planes quite well aligned. 2. EXPERIMENTAL

2.1 Materials Natural graphite flakes from Kropfmiihl near Passau, Bavaria, were used. This material, sieve fraction S 40, has particles with diametersranging from 0.2 to 1 mm. It had been purified to better than 99.9%C by the producer (Graphitwerke Kropfmiihl AG) using a soda melt and a chlorine after-treatment.This graphite is very well crystallized, density (measuredunder xylene) and interlayer distance correspond to the literature values for single crystals: d = 2.253g/cm3; co/2 = 335.41+ 0.06pm. The Br2 intercalation isotherms show distinct steps as is observedonly with defect-free graphites[9]. Commercial graphite foils were used (SigraiIex of SIGRI GmbH, D8901 Meitingen). Sigraflex F 3510had a thickness of 0.35mm, Sigratlex L was 2.0 m m thick. The exfoliated graphite used for the production of Sigraflexis obtained by heating graphite hydrogen sulfate of unknown stage. Since it contains small quantities of sul-

195

1%

L.

STREIFINGER

et al.

furic acid (residue compounds),Sigraflex F 3510 was also used after heat-treatment at 2800°K; this material is designated Sigraflex F (2800°K). The bulk density of Sigraflex is near 1 g/cm3.t 2.2 Preparation

Ampoules were filled in a dry glove box with the graphite, the foils already being cut to size to fit the conductivity cell. After addition of either the calculated quantity or of an excess of the reagent, the reagent was cooled to liquid nitrogen temperature and the ampoules were sealed either at atmospheric pressure or at a vacuum of ca. 1 Pa. The glass tubes were heated for 5-40 days to the desired temperature in an oil bath. Sixty days were allowed for the reaction with SbFSat 20°C. After opening of the ampoules small quantities of adhering reagent were pumped off at reduced pressure (several l@ mbar). If a larger excess of &Cl5 had been used, the sampleswere transferred to a glass frit and washed with a little dry CCL+ All operations were performed under dry nitrogen. Using the dry-box, the material was transferred to the rather wide X-ray capillaries, to the weighing flasks for analysis, and to the cell for the conductivity measurements. 2.3 Analyses

X-ray powder patterns were taken with Cu-K, radiation using Debye-Scherrer cameras with 57mm radius. For chemical analysis, 60-200 mg of the sampleswere decomposed by fusion with sodium peroxide in a Parr bomb. The melts were dissolved in water and filtered from traces of sooty combustion residues. The solutions were neutralized with nitric or sulfuric acid, depending on the ions to be determined,and brought to a volume of 250ml. Aliquots were taken for the individual determinations; a microburette was used for the titrations. Antimony was determined either by bromatometry after reduction with Na*SO, solution and boiling off the SOSexcess, or spectrophotometrically as the Rhodamin B-SbCl- complex[lO]. The results of both methods agreed well with each other. Chloride was determined by potentiometric titration with 0.01 N or 0.1 N AgNO+ Fluoride was titrated with standard La(NO& solution using an Orion fluoride sensitive electrode (against S.C.E.). The point of maximal potential change was taken as end-point. Each determination was repeated at least three times. Carbon was taken as difference to 100%. 2.4 Conductivity measurements

Two measuring cells were constructed from hard polyvinyl chloride. Figure 1 gives the principles of construction. A PVC die with a rectangular opening of 10 m m by 60 or 80 mm, respectively, was surrounded by a steel frame. The PVC pistons fitting the opening were glued to steel plates which fitted into the steel frame. It proved necessary to protect the PVC parts coming into tReagentgrade SbF,, SbCIJ and CC& were obtained from Merck, Darmstadt. Graphite fluoride, CF1.12rwas received from Ozark-Mahoning Co., Tulsa, Oklahoma.

Fig. 1. Cell for resistivity measurements(“exploded” view).

contact with the samples against attack by the Lewis acids by painting them with a thin lacquer film (Tacho Klarlack UVA made by Herberts u. Co., Wuppertal). The PVC surfaces gliding on other surfaces were coated with a Teflon spray. Copper plates covered with platinum foil were placed into the grooves at the small faces of the sample, thus providing contact over the whole cross-section of the compacts. Isolated current leads passed through borings in the steel frame to the copper plates. The probes for the voltage drop were made from copper with tips of 1 m m platinum wire (copper was attacked by the SbCls graphite). They were introduced by holes in the side of the frame, again insulated by Teflon. Their distance was 40 or 60 mm, respectively, in the two cells. The cell was filled with the intercalation compound as evenly as possible to a height of 8-10 m m (resulting in a thickness of the compact of 2-3 mm). The cell was tapped for a while in order to have the flakes orient themselvesparallel to the ground plate as much as possible. After putting the top piston in place, the assembly could be handled in air. The leads were connected, and the cell was put in a hydraulic press equipped with a precision manometer. The sample height was determined by measuring the thickness between the steel plates using a precision gauge. A correction was applied for the change in thickness of the PVC pistons due to elastic deformation by making a blind run with a steel bar in place of the sample. It was in the order of a few tenths of a m m at 600 bar. The limiting pressure was 625 bar because plastic flow of the PVC set in at higher pressures,i.e. 800 bar. It was always checked that both ends of the sample did not differ too much in thickness; if there was a small difference, the average thickness was used. A current of 3-4A was passed through the sample from a d.c. constant current source, it was measured to the nearest mA with a digital multimeter (Data Precision model 134). The potential drop was determined using a precision compensation instrument with a maximal sensitivity of 1pV (Technischer Kompensator of Ruhstrat, Gottingen). The current was passed only for 5-6sec at a time in order to avoid heating of the sample.After final compensation, the result was checked after several minutes waiting time without current. An estimate of possible errors resulted in a maximal relative error of 5%, the largest uncertainty being the

The electric

resistivityof compactsof graphiteintercalationcompounds

197

wide variability in the composition and the structure of the compounds of graphite with SbF5. As is shown below, we found evidence for partial-fluorination of the carbon layers in the reaction with SbFs. The nature of the negatively charged species in the intercalated layers is not known, but very likely it is [SbFJ or [Sb2F1,]-. The carbon layers are oxidized, and hence a reduction product must be formed in the reaction with SbF5. Formation of a Sb3’ species was detected by Ballard et al.[l6] using Mossbauer spectroscopy. It is not known, however, whether this species is intercalated or admixed to the graphite compound. 3. RESULTSAND DISCUSSION Recently, Bartlett et a/.[171 found evidence in the K shell absorption edge spectra for As’+ in the inter3.1 X-Ray data X-Ray powder patterns showed that the products of calation compound with AsF5. It is interesting to note in the reaction with SbCls were a mixture of first and this context, that Boeck and RiidorlT[ll] observed a ratio second stage compounds. The I, values obtained from a halogen/antimony of 4.2kO.l for the 1st stage interseries of (001)reflexes are listed in Table 1. The I, value calation compound with SbF3Clz,with I, varying from for the first stage SbCls compound is 949? 1 pm. This is 842 to 856pm. Mtlin and H&old reported a ratio of slightly larger than the value I, = 942 pm reported by 5.0 ? 0.1 for the SbCl, compound, however[ 1I]. We observed that the first stageof the SbFs compound Melin and HCrold[ll]. This difference is still larger for the second stage (I< = 1272pm in [I I]). Bahnmiiller[l2], is formed already at room temperature (samples No. however, reported that he found variable interlayer dis- 20-22). The graphite foils turned steel blue after two tances for each stage depending on the preparation con- weeks under SbF, without losing their shape and their ditions; he observed increasing I, values with increasing coherence, although cracks formed in the thicker samreaction temperature. The trend observed by us for the ples. Formerly, it was reported that temperatures above second stage is to the contrary, however. 70°C are necessaryfor obtaining the first stage and that Variations in the interlayer distances of the inter- only higher stages are formed at room calation compounds with SbF, were also observed by temperature[l9,20]. It is also noteworthy that the X-ray Bahnmiiller[l2] as well as by us. It is interesting to note patterns of the samples 11 to 13 did not change when the that not two of the I, values described in the literature preparations had been exposed to ambient air for up to agree (Table 2). There is also a considerable scatter of 10 weeks. Freshly cleaved foils intercalated with SbF, the analytic data given in the literature. This is certainly were highly reactive, however. They turned black with not caused by imprecise measurements but by real evolution of fumes upon exposure to ambient air. The differences in the structure. There seems to be a rather blue foils turned black, surprisingly, during irradiation

thickness of the sample. The system was checked by measuring the resistivity of copper: (1.87* 0.01)x IO-’am. One source of error with compacted powders is the unknown porosity of the sample. The results were calculated using the macroscopic dimensions of the compacts. With pure graphite, the sample thickness was also calculated from the mass and the density of the graphite in the cell. The relative pore volume as estimated from the observed thickness was 0.18 at 125bar, it decreasedto 0.026 at 500 bar.

Table I. Characterization Sample No. Intercalation II I2 13 24 23 Intercalation 14

Starting material of SbClc flakes (S40) flakes (S40) flakes (S40) foil Sigraflex F 3S IO foil Sigraflex F (2800°K) of SbF, flakes (S40)

of the materials

used for the resistivity

Reaction temperature

120°C 115°C 200°C 180°C

measurements

Composition

C,P,SbCI,z CM~U~ Cz,&Cl, 7 -

(t

I5 16 19 I7 22 I8 20

flakes (S40) flakes (S40) flakes (S40) foil Sigraflex F (2800°K) foil Sigraflex F (2800°K) foil Sigratlex L foil Sigraflex L

90°C 90°C 90°C 100°C 20°C

-

100°C 20°C

+Minor constituents in parentheses. UZoncluded from the blue color, no X-ray patterns were taken

L]pml

l/2

l/2

95011277 948/ 1278 949/ I270 949/ 1269

l/2

951/1272

112 (I)/2

180°C

70°C

Staget

l/2 graphite) I

SSOll I58

l/O)

833 831 830 833/l I66

I!:

829

C,oS+,,

840

198

L.

STREIFINGER

Table 2. Comparison of the I, values reported by various authors for intercalation compoundsof graphite with SbFr I, values(pm) 2nd stage 3rd stage

Refs.

1st stage

I191 WI

846 1176 1511 1846 797-830 1140-l 160 844+3 117623 1505f 5 1830f 10 1465-1480 1820 1150 I520t 829-840 1158-1166

]31 1131

f141 1151

This work

4th stage

tpresumably3rd stage. with the X-ray beam. Only the parts hit by the beam lost their blue color and new diffraction lines appeared after prolonged irradiation. X-ray studies of the foils were made difficult by this effect. 3.2 Electric resistivity

Higher resistivities are to be expected for polygranular compacts than for single crystals or for dense pyrographites, due to contact resistances. As can be seen from Fig. 2, the resistivity of the compacts prepared from graphite flakes decreased considerably with increasing pressure. The curves do not yet completely flatten out at 600 bar. With the unreacted graphite flakes, the lowest resistivity observed at 625 bar was 261 x 10m8Qm, as compared to pa = 40 x lo-‘Rm for HOPG and p = 1000x lo-‘Rm for graphite electrodes. The resistivity increased to 400 x lo-’ nrn upon release of the pressure. The resistivity of the graphite foils was very little 18 i

etol.

pressure dependent, however. It increased even slightly and irreversibly with pressure (Table 3). The thickness of the foils remained constant within measuring accuracy. It is interesting to note that the graphite foils had a higher resistivity than the compacts of graphite flakes at high pressures. Very likely, some of the edges of the very thin graphite sheets are folded away from the direction parallel to the plane of the foil in the fabrication process, thus giving rise to prism face-to-basal face contacts. This circumstance might also explain the irreversible increase of resistivity upon compression described in Table 3. In addition, the defects caused by deformation and bending of the sheets will increase their resistance. The presence of defects is demonstrated in the X-ray diffractogram: the reflex (112) is slightly broadened as compared to (110). The compacts from graphite flakes had, after pressure relief, a 2.5 times higher conductivity than the foils. The resistivities of the graphite compounds made from foils decreased,however, on applying pressure,albeit to a much lesserextent than with the compactedflakes (Fig. 3). They increased again somewhaton relief of the pressure. The thickness of the intercalated foils was considerably smaller under pressure than one would estimate from the increase in interlayer spacing. Obviously, the pores in the foils can take up a substantialpart of the c-axis expansion; this was most pronounced with the thick foils. As expected, intercalation led to a strong decrease in resistivity. The lowest resistivities observed (Table 4) were well above the resistivity of copper, however. With the best samples, the resistivity fell to 3-4% of that of the parent foil. The resistivity increased, again, on release of the pressure to values observed near 250 bar on compression. There was a relatively wide scatter of the individual runs with preparations from graphite flakes. This is very likely in part due to the differences in packing within the cell. The variations were noticeably larger for the SbFs compounds, however. In a few experiments, current was passedfor extended periods through the foils intercalated with SbClsor SbFs and the change in resistivity due to heating was followed in a qualitative way. All sampleshad at ambient pressure a positive temperature coefficient of resistivity, and behaved like metals. Under pressure, however, the resistance decreasedwith increasing temperature. Contrary to expectation, the compounds with SbC15 had higher conductivities than those with SbF,. Under pressure, the compacts No. 12 and 13 were comparable in their resistivities to the graphite-SbF, foils. After release of the pressure the resistivities of the samples

Table 3. Change of resistivity of graphite foils with pressure I loo

200

300 pressure,

4ccJ

500

600

bar

Fig. 2. Resistivity of graphite and of its intercalation compounds with SbFr and SbC& as function of applied pressure, graphite flakes as starting material. (Graphite S40 -, intercalated with SbClc---, with SbFT-.-.-).

Foil SigraflexF 3510 SigraflexF (2800°K) SigraflexL

Resistivity [am x 10’1 At 1 bar At pressure 1110 880 1060

1180(500 bar) 950 (580bar) 1100(590bar)

The electric Table 4. Lowest Sample No.t

resistivities

observed

lntercalatton of SbCIs II flakes I2 flakes flakes 13 foil 24 foil (HT 2800°K) 23 Intercalation of SbFs I4 flakes flakes 15 I6 flakes 19 flakes foil (HT 2800°K) 17 foil (HT 2800°K) 22 thick foil 18 20 thick foil

of compacts

with the various samples

Pressuret (bar)

Starting material

resistivity

Pmin (flm X IO’= @cm)

380 507 633 591 591

136 68

633 633 481 608 270 591 591 591

182 256 497 125 99 77 135 42

of graphite intercalation

199

compounds

fonic acid[fl], and resistivities as low as 6 x lo-’ fIm were reported for relatively well-crystallized fibers intercalated with nitric acidI221.

3.3 Carbon fluorination

89 36 36

tThe numbers of the samples refer to Table 1 giving characterization. *Final pressure at which lowest resistivity was observed.

their

The IR spectrum of sample No. 15, obtained by intercalation of SbF5, is shown in Fig. 4. The strong peak at 663cm-’ can be ascribed to the SbF,- ion. It would be premature, however, to conclude that this ion is the carrier of negative charge in the intercalate layers. They might as well be secondary products of the reaction of the intercalation compound with the KBr matrix which always contains some moisture. Two weak peaks at co. 1210 and 108Ocm-’are typical for CF and CF2 groups. Graphite fluoride, CF1,r2,gives rise to a strong peak at 1219cm-’ and to a relatively weak peak at 1073cm-‘. Photoelectron spectra of the reaction product with SbF5 are shown in Fig. 5, together with the spectrum of the starting graphite. Curves 2 and 3 correspond to the surface region and the bulk of the intercalated compact, respectively. The bulk spectrum was obtained by cleaving the compact under argon and transferring the material with the fresh, dark blue surface to the sample

3

1400

1200

1000

BOO

600

:m-'

400'

cm-' '23 I loo

200

300

400

500

600

Fig. 4. Infrared spectrum of the intercalation compound of graphite with SbFs, sample I5 (KBr disc technique, PerkinElmer 325 spectrometer).

Fig. 3. Resistivity of graphite intercalation compounds with SbFS and with SbC& as function of applied pressure, graphite foils as starting material. (Intercalated with SbCh -, with SbFJ -.-.-).

No. 12 (with SbCIS)and No. 19 (with SbF5) were 126x IO-’Rm, and 230 x lo-* IIm, respectively. A possible reason for this behavior was found in the results of infrared spectroscopy and XPS studies described below. The lowest resistivity obtained was 36 X lOmERm for SbC&--intercalated foils. This is only slightly better than the u-axis conductivity of graphite single crystals or HOPG. Application of higher pressures might increase the conductivities of the polygranular compacts somewhat, but only little gain can be expected for intercalated graphite foils. Our hope to combine high conductivity with easy handling by using graphite foils was disappointed. The cause lies in the structural defects of the foils. For the same reason, high conductivities cannot be obtained with graphite fibers. A resistivity of approximately 22 x IO-* Rm was recently reported for commercial graphite fibers intercalated with fluorosul-

Fig. 5. Carbon Is photoelectron spectrum of graphite intercalation compound with SbF,, sample 14. (AEI 100 photoelectron spectrometer, AI-K, source. The sample, attached to the sample holder by “silver conductivity paint”, was cooled to liquid N2 temperature). (I), Starting material, graphite flakes S40; (2), exterior surface; and (3) freshly cleaved surface of intercalation compound.

L. STREIFINCER el

200

chamber under Ar protection. The spectra were measured at 77°K. The C 1s peaks of the intercalation compounds are considerably broadened and shifted to higher binding energies. This is caused by fluorination of the carbon layers. Peaks shifted by 2.8 and 5.6eV are attributed to CF groups, and the weak peak at 6.9eV higher B.E. is due to CF2 groups. This is in full agreement with the observations of Cadman et ~I.[231 with fluorinated carbon surfaces, and with the data for fluorinated hydrocarbon polymers [24]. The “bulk” spectrum (curve 3 in Fig. 5) and the “surface” spectrum (curve 2) show that a considerable part of the C 1s intensity is caused by carbon bonded to fluorine. The results of chemical analysis in Table 1 show that most samples had a higher fluorine content than corresponds to a ratio F/Sb = 5. Some of the fluorine excess can be accounted for by the assumption that the negative charge in the antimony fluoride layers is located in the form of SbF6- or SbzF;, ions. Four of the samples analyzed had a F/Sb ratio of near to or higher than 6. However, it seems very unlikely, that all of the intercalated antimony is present in the form of SbF,- ions, becausethis would lead to a very high positive charge on the carbon layers which has never been observed with other intercalation compounds. Obviously, a considerable part of the excess fluorine is covalently bonded to carbon atoms. Only one of the samples analyzed had a F/Sb ratio near 5 (this sample, No. 17, had been prepared from graphite foil). Possibly, the relatively higher resistivities of the foils

Nos. 17 and 18 as compared to Nos. 22 and 20 might be due to the higher reaction temperature (100°C vs 2O”C), leading to a higher fluorination of the surface or of the

carbon layers. However, sample 17 could be measured only up to 270 bar. If there was a substantial carbon fluorination, sample 17 should contain some Sb3+ (sample 18 could not be analyzed, unfortunately). All these observations indicate that at least a considerable part of the excess fluorine is bound to carbon atoms of the layers. Apparently, the outermost carbon layers and/or the edges at the prismatic crystal faces are

similar to graphite fluoride. White graphite fluoride with a F/C ratio of 1 is an insulator, and the black preparations with F/C < 1 have conductivities

lower than that of

graphite. This surface fluorination of the SbFs intercalation compound

leads thus to an increase in the

interparticle resistances. It is to be remembered that SbF, is used as a fluorinating agent in organic chemistry. The fluorinating properties are much stronger for CIF,, and pronounced C-F signals were observed in the IR as

a/.

Evidence for carbon fluorination has recently been described by Selig et al. for the intercalation products with xenon fluorides or with AsF5[26]; CF,’ peaks were observed in the mass spectra during pyrolysis. It was not clear from these observations whether C-F bonds are formed already at or below 100°C.We have shown that this is indeed the case. Acknowledgements-We are indebted to SIGRI Elektrographit

GmbHfor help with materialsand workshopfacilities.Financial support by Fonds der chemischen Industrie is gratefully acknowledged. REFERENCES

1. A. R. Ubbelohde, Proc.

Roy.

(1%8);Carbon 14, 1 (1976). 2. F. L. Vogel,Bull. Am. Phys.

Sot.

(London)

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Sot. 21, 262 (1976); J. Mat. Sci.

12,982 (1977).

3. T. E. Thompson, E. R. Falardeau and L. R. Hanlon, Carbon 15. 39 (1977).

4. H. Fuzellier, J. Melin and A. H&old, Carbon 15, 45 (1977). 5. C. Zeller, G. M. T. Foley, E. R. Falardeau and F. L. Vogel, Mot. Sci. Engng 31,255 (1977). 6. E. R. Falardeau.G. M. T. Foley, C. Zeller and F. L. Vogel, J. Chem. SOL, Chem. Comm.

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7. G. M. T. Foley, C. Zeller, E. R. Falardeau and F. L. Vogel, So/id-St. Comm. 24, 371 (1977). 8. F. L. Vogel, G. M. T. Foley, C. Zeller, E. R. Falardeauand J. Gan, Mat. Sci. Engng 31, 261 (1977). 9. I. Pflugmacher and H. P. Boehm, Extended Abstracts, 10th Biennial Conference on Carbon, Bethlehem, PA (1971). p. 259.

IO. J. Fries and H. Getrost, Organische Reugenzien fir die Sourenanalvse. E. Merck, Darmstadt (1975)D. 33. 1I. J.‘MClin and A. H&old, Carbon 13, 3i7 (l97$. 12. H. Bahnmiiller, Dissertation, University of Tiibingen (1976). 13. L. B. Ebert, R. A. Huggins and J. I. Brauman, .I. Chem. Sot., Chem. Comm. 1974,924. 14. S. C. Singhal, J. Schreurs and R. C. Kuznicki, Mat. Sci. Engng 31, 123 (1977).

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Russ. J.

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16. J. G. Ballard and T. Birchall, J.

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son and A. C. Thompson, J. Chem.

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18. A. Boeck and W. Riidorff, 2. Anorg. Al/g. Chem. 384, 169 (1971). 19. 1. Melin and A. H&old, Compt. Rend. Acad. Sci. France zsoc, 641 (1975). 20. Lin Chun-Hsu, H. Selig, M. Rabinovitz, I. Agranat and S. Sarig, Inorg. Nucl. Chem. Letters 11, 601 (1975). 21. J. 0. Besenhard,H. P. Fritz, H. Miihwald and J. J. Nickl, Z. Naturforsch.

33b. 737 (1978).

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(1977). 24. J. J. Pireaux, J. Riga, R. Candano,J. J. Verbist, J. M. Andre, J.

reaction products with this compound. Compacts of these preparations were not electrically conducting at all, apparently because a surface layer of graphite monofluoride had formed[25].

Delhalle and S. Delhalle, J. Elec. Spectr. 5, 531 (1974). 25. L. Streifinger, Diplomarbeit, University of Miinchen (1978); to be published. 26. H. Selig, M. J. Vasile, F. A. Stevie and W. A. Sunder, J: Fluorine Chem. IO, 299 (1977).