1322 Diamond and Related Materials, 2 (1993) 1322-1326

Study of high pressure diamond synthesis by M6ssbauer spectroscopy*

L. S. de Ol iveira Universidade Federal de Santa Catharina, 88000 Florianopolis, SC (Brazil)

J. A. H. da J o r n a d a Universidade Federal do Rio Grande do Sul. 91540 Porto Alegre. RS (Brazil)

(Received August 31, 1992; accepted in final form November 16, 1992)

Abstract

Graphite samples containing small amounts of STFe (less than 0.3 at.%) were processed under pressures and temperatures normally used for diamond synthesis and subsequently analyzed by M6ssbauer spectroscopy. The use of small quantities of Fe enables a better study of Fe solubility in graphite and the compounds eventually formed around the graphite-Fe interface. The results show basically the same superficial compounds observed in carbon supported iron catalysts, indicating a very small solubility of Fe in graphite (less than 0.03 at.%). When water was present in the reaction cell, the spectra show iron hydroxide and Fe-C hydrated compounds, which can explain the deleterious effect of water on diamond synthesis by disturbance of the Fe-C interface.

1. Introduction

Despite many studies, there is no clear understanding of the microscopic mechanisms involved in high pressure diamond synthesis, especially the nucleation process. Besides the unlikely possibility of homogeneous nucle- ation, different physical mechanisms have been proposed to explain nucleation, involving the formation of inter- mediate carbides [1, 2], the formation of a coloidal solution of graphite in the metal catalyst [-3, 4], and the intercalation of catalyst atoms between the graphite planes [-5, 6].

The roles of different graphite samples and residual gases on the reaction cell, especially water and hydrogen are also unclear. It is known that water can be very deleterious to diamond formation [7], and that the existence of adsorbed gases in graphite can reduce the diamond yield to null values I-8]. There is no satisfactory explanation for the process yet.

As iron is a very important solvent-catalyst for dia- mond synthesis, and as M6ssbauer spectroscopy is especially convenient for iron, the study of diamond synthesis using this technique seems very interesting. However, as the most interesting phenomena should occur near the Fe-graphite interface, the presence of large amounts of iron overwhelms the spectrum of iron atoms in this region.

In the present work, we used very small amounts of

*Paper presented at Diamond 1992, Heidelberg, August 31-September 4, 1992.

enriched 57Fe to study details of diamond synthesis, especially the possibility of Fe intercalation (or dissolu- tion in graphite) and the role of water present in the reaction cell. Some preliminary results have already been presented in a preceding paper [9].

2. Experimental details

Very small amounts of enriched iron (95.5% 57Fe), as very fine powder of Fe20 3 dispersed in alcohol, were painted on discs of graphite (purity 99.99%). This was followed by a reduction process under argon atmosphere at 1000°C. The M6ssbauer spectrum showed ~-Fe and minor amounts of Fe-C solid solution. The average total concentration of Fe in graphite was less than 0.3 at.%.

The graphite samples were then treated under high pressure, in a belt chamber, at a pressure of 55 kbar and a temperature of 1500 °C, which is near to the conditions normally used for diamond synthesis with iron as catalyst.

In order to detect the effect of water, two environments were used: talc, which releases water upon heating, and boron nitride (BN), an anhydrous material.

Sample A was processed during 20 min with BN, sample B during 10 min with BN, and sample C during 10 min with talc.

In order to subject the system to much more water, another sample, D, was processed three times succes- sively, each time for 10 min in a new talc cell.

Elsevier Sequoia

L. S. de Oliveira, J. A. H. da Jornada ,; M6ssbauer study of diamond synthesis 1323

After the treatments, the samples were analyzed by M6ssbauer spectroscopy using conventional equipment with a 57Co source in an Rh matrix. The spectra were taken at room temperature (RT) and at 77 K (liquid nitrogen temperature LN), and for sample B a spectrum at 4 K was also recorded.

3. Results and discussion

Typical M6ssbauer spectra are displayed in Figs. 1 and 2. In sample A, the spectrum shows basically two quadrupolar interactions, one small with AEQ character- istic of an Fe 3 + site, and another compatible with an Fe z + site with large value of AE o. The latter component is in fact composed of two sites with slightly different AEQ as can be seen in the measurements of sample D at LN temperature, Fig. 2. In all samples no magnetic splittings were observed at LN temperature. For

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Fig. l. M6ssbauer measurement at R of sample A (processed during 20 min with BN) and sample B (processed during 10 min with BN), and sample B measured at 4 K.

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Velocity (mm/s) Fig. 2. M6ssbauer measurement at RT of sample C (processed during l0 min with talc) and sample D (processed with talc), and sample D measured at LN temperature.

sample B, which was processed in a shorter time (10 min), Fe 2÷ was practically absent. However, for sample C, also processed for 10 min but in a talc cell, the presence of the Fe 2÷ components was greatly increased. For sample D, which was exposed to a much greater water environment, the result shows only the Fe 2÷ component, the Fe 3÷ component being totally absent.

The quadrupolar splitting and isomer shift (IS) for the observed spectra together with some literature results for comparison are shown in Table 1.

The Fe 2 ÷ compounds in samples A and C are practi- cally the same as obtained when a larger amount of talc was used in the reaction cell (sample D), however, as can be seen in Fig. 2, the spectrum at 77 K clearly shows two doublets, indicating two sites, with close hyperfine parameters. The Fe 2÷ site with smaller intensity, in sample D, whose hyperfine parameters at 77 K are AEQ= 3.04 __ 0.11 mm s -1 and IS=1.28 +0.11 mm s -1

1324 L. S. de Oliveira, J. A. H. da Jornada / Mi~ssbauer study of diamond synthesis

TABLE 1. M6ssbauer parameters for the present study and for some F-C-O-H compounds: IS is given in relation to ~t-Fe 1-10-12]; errors are of the order of +0.11 mm s -1

Compound a T (K) AEQ (mm s- 1) IS (mm s- 1)

Fe 2+ RT 2.12 1.17 77 2.17 1.28

Fe 2 + 77 3.04 1.28 Fe 3 + RT 0.71 0.37

77 0.77 0.48 Fe(OH)2 78 3.08 1.25 Fe(C204)(H20)2 RT 1.75 1.21

100 1.98 1.23 50 2.06 1.26

Fe(C404)(H20)2 RT 2.29 1.22 50 2.79 1.31

aFe2+ parameters obtained from sample D, and Fe 3+ parameters obtained from samples A and B of this work.

is identified as Fe(OH)2 since these parameters are very close to those found in the literature for this compound (see Table 1). The other component, with AEQ= 2.17 + 0.11 mm s- 1 and IS-- 1.28 + 0.11 mm s- 1, is very probably due to i ron-carbon-oxygen compounds such as iron oxalate (FeCEO4(H20)2) or iron squarate (FeC404.(H20)2), both with hyperfine parameters shown in Table 1. As can be seen, the agreement with the literature values is fairly good, the small discrepancies being attributed to the fact that the compounds are chemically complex and possibly formed in the graphite surface as very small particles and with inexact stoichi- ometry. This may lead to the small differences in IS and AEQ [13]. In addition, Champion et al. [14] reported that AEQ for iron oxalate increases with pressure and may remain constant after pressure release.

The presence of these types of compounds can be directly associated with the presence of water [9], since talc releases chemical water during high-pressure and high-temperature treatment [15], allowing the formation of hydrated compounds; Some' adsorbed water could also come from the external parts of the cell which were made from pyrofilite. This can explain why the sample processed for 20 min (A), shows some Fe 2 + components despite the use of BN (see Fig. 1). The existence of these compounds for both processes could explain the deleteri- ous effect of water on diamond synthesis observed by Kanda E7], by disturbance of the direct contact between metal and graphite.

The Fe a + compounds, whose M6ssbauer parameters are AEo, =0 . 71mm s -x, ISRT=0.37mm S -x and AEQLN=0.77 mm s -1, ISLN=0.48 mm s -x are basically the same as observed in experiments with industrial catalysts, consisting of iron supported on carbonaceous materials, in the studies conducted by Jung et al. [16] and Niemantsverdriet et al. [17], where the parameters were AEQ, T =0.74 mm s - ! and ISRx=0.38 mm s-x. In

those studies, the spectra were attributed to a kind of iron oxide, finely dispersed at the surface.

The results for the measurements of sample B at 4 K showed another important feature. In Fig. 1, magnetic splittings, which contribute about 30% to the total area of the spectrum, can be observed at 4 K. Despite limita- tions in accuracy due to poor statistics, two values of hyperfine magnetic fields are derived: 346kOe (19%) and 382 kOe (10%). The first could be attributed to metallic iron and the second to some iron oxide, both with very fine particle sizes. Again, this result is similar to the studies already mentioned on catalysts, where it was observed that when the sample was protected from air after preparation, a superparamagnetic spectrum of metallic iron was observed which splits into a magnetic sextet at 4 K with a hyperfine field of 345 kOe [17]. However, when it was exposed to air, a magnetic sextet at 4 K attributed t o iron oxide appears, owing to the spontaneous oxidation of such small particles.

This similarity probably means that in the present case the iron particles were very small (less than 20 A) (showing superparamagnetic behaviour [13, 17, 18]), but were not fully oxidized when the sample was put in contact with air, as occurred with the catalysts. This can be explained if it is considered that in the high-pressure high-temperature process, a certain amount of very fine iron particles remains encapsuled in microporosities or grain boundaries in the polycrystalline graphite and is therefore inaccessible to oxygen.

From the above discussion, one can see that all the components of the spectra for the different samples are fairly well explained by known compounds, which are conceivably formed under the different conditions of this work. There is no indication of unexplained components, which would indicate possible dissolution or even inter- calation of Fe atoms in graphite.

Considering in the present work the impossibility of determining components contributing less than 10% to the total area of the spectra, the solubility of iron in graphite should be less than 0.03 at.%. In previous studies of the Fe -C system with very low amounts of iron, prepared by different means such as evaporation [19], ion implantation [20] and for the above mentioned catalysts [16, 17], it was found that iron combines with superficial carbon forming compounds which remain agglutinated at the surface or segregated at the graphite grain boundaries in clusters. These results, and the high pressure results of the present work, allows us to con- elude that the solubility of iron in graphite is in fact very small, which is at variance with the value of approximately 0.3 at.% Fe showed in a phase diagram by Strong [21].

One can make a rough estimate of the energy neces- sary to introduce an iron atom into graphite AE using

L. S. de Oliveira, J. A. H. da Jornada : M6ssbauer study of diamond synthesis 1325

the solubility at a temperature T given by [-22]

c ~ e x p ( - A E / k T )

Assuming for the present case c<0.03 at.% at T 2000 K, one finds AE > 1.4 eV. This value is in fact much lower than is expected [23]. A possible solution of Fe in graphite would involve Fe occupying a substitutional site in the graphite planes or an interstitial site between the graphite planes, and both cases are very expensive in terms of energy. For the substitutional position, the energy necessary only to produce a vacancy in graphite is around 7.0 eV. The other possibility, i.e. that the Fe occupies an interstitial position between the carbon planes, is also difficult because the energy due to the lattice distortion should be very large.

Those considerations, together with the negative results of this work for an indication of the dissolution of Fe in graphite, render unlikely the hypothesis of Kalashnikov et al. [-6] that the diamond nucleation process may be due to the intercalation of Fe in the graphite planes.

4. Conclusions

This work presented a M6ssbauer study of the F e - C system under high pressure and high temperature in the diamond synthesis region, with samples consisting of graphite with very low iron concentration (0.3 at .% of 95.5% 57Fe).

The very low iron concentration and the M6ssbauer spectroscopy allowed the study of microscopic processes at the graphi te-metal interface, under the conditions of diamond synthesis. It was observed that in this region, compounds can be formed on the graphite surface or that iron atoms are possibly introduced into the graphite bulk as small clusters, being situated at grain boundaries or microporosities which are always present in polycrys- taline graphite.

The presence of water allows the formation of hydrated compounds of carbon and iron at the graphite-iron boundary, and this could explain the delete- rious effect of water on diamond synthesis [-7]. Possibly, the compounds formed at the interface prevent the metal - graphite contact necessary for the transformation. This contradicts the mechanism proposed by Polyakov [24] in which the competition in solubility between hydrogen and carbon in iron is responsible for the observed effect of hydrogen, i.e., hydrogen with a higher solubility than carbon in the metal would hinder the necessary introduc- tion of carbon atoms into the metal.

All of the components of the M6ssbauer spectra observed in this study were identified with known com- pounds which may plausibly be formed during the treatments used, and there was no evidence of different

components which could indicate Fe atoms dissolved or intercalated in graphite. Therefore, the possibility of intercalation in graphite layers being responsible for the nucleation of diamond seems unlikely. Also, the solubil- ity of Fe in graphite is estimated to be lower than 0.03 at.%, which is much lov~er than the value depicted in the phase diagram by Strong [21].

Although the conclusion of this study is strictly applic- able only to iron, it could be considered more general because of the chemical similarities between the trans- ition elements used as solvent-catalysts for diamond synthesis.

The use of direct microstructure investigation tech- niques such as transmission electron microscopy and Auger spectroscopy seems necessary as a complementary study of the compounds formed, and especially to clarify the nature of the interface.

Acknowledgments

We thank M. T. X. Silva for the M6ssbauer measure- ments and helpful discussions. This work is supported in part by CNPq, F I N E P and FAPERGS.

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