Synergistic Effect of Fluorine and Hydrogen on Processes of Graphite

Sep 20, 2011 - ... by high yield of planar graphite monocrystals of clear-cut polygonal shape. ... Another distinctive feature of diamond formation pr...
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Synergistic Effect of Fluorine and Hydrogen on Processes of Graphite and Diamond Formation from Fluorographite-Naphthalene Mixtures at High Pressures Valery A. Davydov,† Aleksandra V. Rakhmanina,† Vyacheslav N. Agafonov,‡ and Valery N. Khabashesku*,§ †

L. F. Vereshchagin Institute for High Pressure Physics of the RAS, Troitsk, Moscow region, 142190, Russia L.E.M.A., UMR CNRS-CEA 6157 - LRC CEA M01, Universite Franc-ois Rabelais, av. Monge 31, Tours, 37200, France § Department of Chemical and Biomolecular Engineering, University of Houston, 4800 Calhoun Rd, Houston, Texas 77204, United States ‡

ABSTRACT: Pressure temperature-induced transformations of fluorographite CF1.1 and homogeneous mixtures of CF1.1 with naphthalene under static pressure of 8.0 GPa have been investigated by X-ray diffraction, scanning and transmission electron microscopies, and energy-dispersive X-ray analysis. It was found that carbonization of neat CF1.1 occurs at temperatures above 500 °C. Formation of graphite showing high grade of crystalline perfection has been observed already at ∼900 °C. The process of graphitization of carbon residue from CF1.1 is also characterized by high yield of planar graphite monocrystals of clear-cut polygonal shape. The formation of diamonds from CF1.1 at 8.0 GPa was not observed within all studied temperature range, up to 1500 °C. Thermal transformations of CF1.1 mixtures with naphthalene are distinguished by significantly reduced graphitization temperature threshold and a record low initiation temperature of diamond formation, 900 °C at 8.0 GPa, in comparison with various hydrocarbons. This can be explained by synergistic effect of fluorine and hydrogen on processes of graphite and diamond formation in a binary system studied. Another distinctive feature of diamond formation process in the systems composed of mixtures of CF1.1 and naphthalene is simultaneous production of both nano- and micrometer-sized diamond fractions.

1. INTRODUCTION Studies of thermal transformations of carbon-containing materials under high pressures are of significant interest, from both fundamental and applied points of view. The ongoing research efforts are driven by substantial modifying effect of pressure on complex sequence of physicochemical transformations, which involve carbonization of carbon compounds and subsequent structural assembling of carbon residue at high temperatures. Pressure significantly affects the temperature parameters of these transformations as well as structure and properties of the resulting products. The goal of present work was studies of thermal transformations of fluorocarbon materials and their binary mixtures with hydrocarbons under high pressures. Herein, fluorographite CF1.1 and binary mixtures of CF1.1 with naphthalene C10H8 have been chosen for investigations. Unlike fluorocarbon materials, of which carbonization and graphitization under high pressures are barely explored, the processes of thermoinduced transformations of hydrocarbons under high pressures are studied in detail. These studies include examples of both model individual compounds with different molecular structures1 10 and also natural mixtures of hydrocarbons with complex compositions.11 14 For instance, these processes have been extensively studied for naphthalene and other polycyclic aromatic compounds.5,8,9 It was shown that thermal treatment of these polycyclic compounds under pressure results in chemical decomposition yielding a solid carbon residue and volatile hydrocarbon fractions. Treatment at elevated pressure and steady temperature produces increased yield of solid residue. r 2011 American Chemical Society

In this case, structure of carbon residue is controlled by pressure, temperature, time of isobaric treatment, and extent of leak tight sealing of reaction zone. At pressure of 8 GPa, temperatures of 500 600 °C, and times of isothermal treatment as short as just a few minutes, carbon residue is formed as a mixture of two-dimensionally ordered and disordered carbon. At temperatures of 700 800 °C, turbostratic packings of graphene layers separated by characteristic interlayer distances in the range of 0.340 to 0.344 nm are formed. Further increase in treatment temperature results in 3D ordering of graphene layers, annealing of defects, and formation of graphite structure. At pressure of 8 GPa, clear signs of initiation of 3D ordering of graphene layers are observed at temperature of 900 °C. At temperature of 1150 °C, the whole bulk of carbon material adopts a structure of perfect graphite with crystal sizes in the range of 5 20 μm. Under treatment temperatures of 1300 °C and above, microcrystalline diamond becomes a major product of transformation.8 It was also found that at 8 GPa and 1300 °C, bulk diamond is produced not only from carbon residue, formed through carbonization of naphthalene, but also from graphite being in contact with carbon residue.8 These experimental data show that the processes of bulk diamond formation in a hydrocarbon system under high pressures proceed through graphite formation step. It has been noticed that the presence of even a relatively small amount of hydrogen, remaining in the system during final stages Received: July 19, 2011 Revised: September 19, 2011 Published: September 20, 2011 21000

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The Journal of Physical Chemistry C of carbonization of polycyclic aromatic compounds, is capable of catalyzing the transformation of graphite into diamond under considerably lower pressures and temperatures than p,T parameters for direct graphite diamond phase transition.15,16 Although the specific mechanisms of hydrogen participation in the processes of diamond formation from graphite in the hydrocarbon systems under high static pressures are not unambiguously established, it is obvious that they involve chemical interaction of hydrogen with graphite, that is, hydrogenation of graphite. The important result of this interaction is the transformation of carbon atoms from sp2 to sp3 hybridization state, which facilitates the formation of diamond seeds. Possible mechanism for nucleation and continuing growth of diamond under conditions of hydrogenation of edge atoms in graphite crystallites has been proposed.17 According to this mechanism,17 the shaping of diamond can proceed through initial formation of diamond seed from sp3 state carbons, located on edges of hydrogenated graphite particles. This is followed by subsequent growth of diamond particles by pushing a graphite diamond interfacial boundary, which represents a hydrogenated transition layer with distinct structure, into a volume area of graphite particle. In this case, structuring of three (111) planes of diamond is realized through folding and interlayer crosslinking of two (0001) graphite planes. The proposed mechanism agrees well with the results of experimental studies of diamond formation under high pressures in a hydrocarbon system6,8 and explains how a relatively insignificant amount of hydrogen can catalyze the observed bulk transformation of graphite into diamond. Nevertheless, by taking into account significantly higher hydrogen etching rate of graphite than diamond and also the usually larger size of diamond particles produced from hydrocarbons than graphite particles formed by graphitization of the same hydrocarbons,8 it can be suggested that the role of hydrogen is not limited only to “catalytic” effect on the system. It is quite possible that hydrogen participates in the gas-phase (fluid) transport of carbon atoms to growing edges of diamond in the form of light hydrocarbon fractions and reactive free radicals, for example, methyl radicals, playing an important role in the processes of diamond synthesis by CVD.18 20 This should facilitate a sufficiently high level of mobility for carbon atoms at temperatures, relatively low for carbon system. Therefore, available literature data clearly show that hydrogen can play significant role in high-pressure high-temperature-induced transformations of hydrocarbon materials. In present work, besides studies of carbonization of fluorocarbon material under high pressure, the effect of fluorine on processes of graphitization of carbon residue and formation of diamond have been investigated. As known,21 24 the introduction of any form of fluorine into gas mixtures used as precursors in diamond synthesis by CVD facilitates a notable reduction of initiation temperature of diamond formation. In this work, we evaluated the effect of fluorine on initiation temperature threshold for graphitization and formation of diamond under high static pressures. We have studied the processes of thermal transformations under high pressures in a neat fluorocarbon and mixed fluorine hydrogen carbon systems using fluorographite CF1.1 and binary mixtures of CF1.1 with naphthalene as examples. Choice of fluorographite CF1.1 as an object of present studies was primarily based on distinctive structure of this material, very interesting from viewpoint of diamond synthesis in a nonmetal growth systems. It is known25 27 that the structure of CF1.1

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Figure 1. Fragment of fluorographite CF1.1 structure.

represents a packing of weakly interacting fluorocarbon layers having their loading frame built from cyclohexane ring sp3 state carbons (Figure 1). Fluorine atoms covalently bonded to carbon atoms form two sublayers that are located above and below each carbon layer. Therefore, it is noted that the CF1.1 structure originally contains folded layers of sp3 carbon that represent the layers of diamond structure. Compression of the packings of these layers under high pressure with simultaneous removal of fluorine under defined conditions could perhaps lead to cross-linking of adjacent layers of sp3 carbon atoms and result in the formation of diamond structure. Studies of the viability of such diamond formation mechanism in both neat fluorine carbon and mixed fluorine hydrogen carbon systems have been attempted in present work. It was expected that in the first case cleavage of fluorine carbon bonds will be caused by direct thermal excitation of C F bonds; in the second case, fluorines that break off the surface of carbon layers should be additionally facilitated by exothermal chemical interaction of detached fluorine and hydrogen atoms producing thermodynamically very stable molecules of hydrogen fluoride (HF). Given that, we tried to evaluate the effect of structure of starting carboncontaining material on character of carbonization and p,T parameters of graphitization and formation of diamond under high pressures.

2. EXPERIMENTAL SECTION In this work, we have carried out comparative investigation of structural evolution of carbon materials resulting from thermobaric treatment of fluorographite and its binary mixtures with naphthalene at 8 GPa and different temperatures ranging from 20 to 1500 °C. As starting materials, we used fluorographite in the form of refined white-colored powder of CF1.1 composition received from Aldrich Chemical and naphthalene (C10H8) with the impurity content 500 °C. When treatments are done at temperatures >900 °C, the mass loss reaches a steady value of ∼76.6%. In this case, fluorine content drops to near-zero, being at the level of 0.1 to 0.3 at %. Bearing in mind that fluorine content in starting CF1.1 is 52.4 at % or 63.5 wt % accordingly, the observed mass loss value by samples at temperatures >900 °C indicates that besides fluorine a distinct quantity of carbon atoms is also leaving the system. The calculated F/C weight ratio for evolving products, 63.5:13.1, corresponds to 3:1 atomic F/C ratio. The smallest fluorocarbon molecules matching this stoichiometry are trifluoromethyl radical CF3• and hexafluoroethane C2F6, although other combinations of more complex gaseous products of CF1.1 decomposition can also be a match. XRD patterns, shown in Figure 2, provide data on evolution of the structure of carbon residue in relation to treatment temperature of starting CF1.1. They show that active stages of carbonization of CF1.1 through formation of volatile products and solid carbon residue, start at temperatures above 500 °C. XRD pattern of the sample treated at 600 °C shows four broad diffraction peaks at 2θ = 30.5, 50.5, 63.4, and 93.8° and elevated background at smaller angles. Qualitatively, this type of diffractograms points at the presence in the sample of layered ordered and disordered carbon fractions. Diffraction lines at 2θ = 30.4 and 63.4° correspond to (002) and (004) reflections, which result from stacks of parallel carbon layers. Bands at 50.5 and 93.8° are related to 2D (10) and (11) reflections from 2D-ordered carbon lattices.29 Presence of (002) and (004) reflections in the absence of 3D (hkl) reflections suggests that the ordered carbon fraction in the product is represented by packings of distinctly ordered

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graphene planes in the direction perpendicular to the plane. However, these packings lack 3D crystalline ordering. Substantial background level in the low-angle region of diffractogram is affected by the presence of appreciable content of disordered carbon in the sample.31 According to X-ray data, the mean value of interlayer distance (d002) between graphene layers in the packings is equal to 0.340 nm. In this case, mean values of lateral size of graphene layers (La) and height of packings (Lc), estimated on basis of Scherrer equation,28,29 were found to be 5.0 and 3.5 nm, respectively. Comparison of these values with mean particle sizes of starting CF1.1 led to the conclusion that treatment of starting material at 8 GPa and 600 °C does not significantly change the lateral size of particles; however, loss of fluorine is accompanied by a notable thinning of layered packings. In general, the diffractogram of obtained carbon material resembles the diffraction patterns of materials produced in intermediate stages of carbonization of hydrocarbons under high pressures. Nevertheless, it should be noted that in the case of carbonization of polycyclic aromatic hydrocarbons at 8 GPa, the formation of this type of material has been observed at higher temperatures (∼800 °C).8 SEM images of samples produced at 8 GPa and 600 °C (Figure 5) show agglomerates of individual particles lacking a clearly featured morphology. Increase in treatment temperature results in successive narrowing of diffraction peaks, shifting of peak maxima corresponding to (002) and (004) reflections toward larger angles, and decline of the background level in a small-angle region. Such changes in the XRD patterns are related to increases in mean sizes of graphene layers and thicknesses of their packings, shortening of interlayer distances in graphene packings (Figure 4), and reduction of the content of disordered carbon in the samples. In the diffractograms of samples, obtained at 900 °C and higher temperatures, appearance of 3D (100), (101), and (110) reflections is observed. This gives evidence of emergence of 3D ordering of graphene planes and formation of graphite. Figure 4 shows temperature dependence of interlayer distances (d002) of carbon materials produced by thermal transformations of CF1.1 and naphthalene8 at 8 GPa. Dashed line in Figure 4 corresponds to d002 value of perfect graphite, 0.3354 nm. The presented data show that in the case of CF1.1 the same degree of graphitization of carbon residue is achieved at lower treatment temperatures than in the case of naphthalene. For instance, as shown in Figure 4, carbon residue from treatment of CF1.1 at temperature of 900 °C adopts a structure closely resembling that of perfect graphite, whereas during treatment of naphthalene the similar state is reached at temperature as high as 1100 °C. SEM images of samples, produced after treatment of CF1.1 at 8 GPa and 900 and 1100 °C (Figure 5d,e), show that graphite particles of micrometer size become main products of transformations under these conditions. It should noted that graphite particles formed during the process of carbonization of fluorocarbon system possess high content of graphite monocrystals showing clearly shaped polygonal form. This special feature is interesting to point out because graphite particles produced by thermal transformations of naphthalene and other hydrocarbon compounds at 8 GPa usually show oval shapes (Figure 5f). At the same time, these studies have shown that unlike naphthalene, thermal treatment of CF1.1 at 8 GPa does not result in the formation of diamond up to temperature of 1500 °C. This led to the conclusion that the anticipated mechanism of diamond formation through interlayer polymerization of folded carbon layers under conditions of high-pressure carbonization of 21004

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Figure 7. SEM images of samples produced by treatment of 1:1 homogeneous mixtures of CF1.1 with naphthalene at pressure of 8 GPa and different temperatures: 500 (a), 600 (b), 800 (c), 900 (d), and 1100 °C (e).

CF1.1 does not occur. The other proposed mechanism of diamond formation through fluorination of carbon atoms in edge positions of graphite crystallites, taking place during their hydrogenation in hydrocarbon systems,17 is not getting realized either. A possible reason for that is substantial difference in size and chemical nature of fluorine and hydrogen atoms leading to structure perturbation of graphite-diamond transition layer, which is essential for diamond growth by this mechanism in a hydrocarbon system, where hydrogen is replaced by fluorine. 3.2. Thermal Transformations of Homogeneous Mixtures of CF1.1 and Naphthalene at 8 GPa. Evolution of XRD patterns

of products resulting from treatment of homogeneous mixture of CF1.1 and naphthalene (with mass ratio CF1.1/C10H8 1:1) under pressure of 8 GPa and variable temperatures can be observed in Figure 6. SEM images of samples produced by treatment of this binary mixture at different temperatures are shown on Figures 7 and 8. Analysis of these data shows that active stages of carbonization processes in binary mixtures of CF1.1 with naphthalene involve the release of fluorine and hydrogen, and subsequent structural assembling of carbon residues. They proceed at significantly lower temperatures than in cases of neat CF1.1 and naphthalene. 21005

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Figure 9. TEM images of nanosize diamond crystals produced by treatment of 1:4 homogeneous mixtures of CF1.1 with naphthalene under pressure of 8 GPa and temperature of 1000 °C.

Figure 8. SEM images of diamonds produced by treatment of 1:4 homogeneous mixtures of CF1.1 with naphthalene at pressure of 8 GPa and temperature of 1000 °C.

This can be clearly noticed by comparison of XRD patterns of products resulting from treatment at 8 GPa and 500 600 °C of CF1.1 (Figure 2), naphthalene (Figure 1 from work8), and their binary mixtures (Figure 6). According to data presented on Figures 2 5, fluorographite CF1.1 converts at 500 °C into a material that is produced at initial stages of carbonization of CF1.1. A significant part of this material is still composed of CF1.1, whose major reflections dominate the XRD pattern of the sample produced at 500 °C. Fluorine loss is 800 °C. Difference in morphologies of the products of transformation of neat CF1.1 and binary mixture of CF1.1 with naphthalene can be observed on SEM images of these materials (Figures 5b and 7a). According to Figure 5b, the material resulting from treatment of CF1.1 represents an agglomerate of small particles lacking any particular morphological features, whereas the material obtained from binary mixture (Figure 7a) forms a separate flakes typical for layered graphene packings. Such significant difference in formation temperature for the same carbon states obviously points at certain dissimilarities in the mechanisms of carbonization of neat hydrocarbon and fluorocarbon compounds and their binary mixtures. In our point of view, a drop in graphitization temperature threshold for CF1.1 and naphthalene in binary mixtures, noted in this work, is due to difference between purely thermal and combined thermochemical mechanisms of carbonization of fluorocarbon and hydrocarbon compounds. The obtained results led to the conclusion that the processes of defluorination and dehydrogenation of neat CF1.1 and naphthalene, respectively, is mainly thermally activated. In the case of binary mixtures, there appears to be an

additional thermochemical channel for processes of carbonization of both components of the mixture, which involves an active fluorine hydrogen chemical interaction leading to thermodynamically favorable formation of HF molecules and therefore requiring lower activation energy. The outcome of this mechanism is an effective drop of temperature threshold for graphitization of carbon residue produced by carbonization of CF1.1 and naphthalene binary mixtures. The emergence of 3D ordering of packings of graphene layers and formation of graphite is indicated in XRD by a modulation of 2D (10) reflection near 50°, which in the case of binary mixture becomes observable after treatment already at temperature of 600 °C. Treatment of binary mixture at 700 and 800 °C produces graphite materials that form crystals with the degree of perfection increasing with temperature (Figures 6 and 7). As SEM images show, consistent elevation of treatment temperature is followed by gradual increase in lateral size and thickness of graphite crystallites. According to Figure 7d, the lateral size of graphite particles at temperature of 900 °C can reach 5 10 μm. Along with graphite, the formation of diamond, showing a clear (111) reflection on XRD in Figure 6, is also observed at this temperature. The appearance of submicrometer- and micrometer-size diamond crystals is presented in Figure 7d. At treatment temperatures >1000 °C, diamond becomes a major component of the products of transformation of binary mixture of CF1.1 with naphthalene (Figures 6 and 7e). Although the formation of diamond has been observed for all studied compositions of binary mixtures of CF1.1 with naphthalene, the content of diamond fraction in the transformation products obtained under the same treatment temperature definitely depends on atomic F/H ratio in the starting mixture. In the series of experiments on different mixture compositions of CF1.1 and naphthalene with atomic F/H ratios of 1:1, 1:2, 1:3, 1:7, and 1:10, carried out at 8 GPa and temperature of 1000 °C, maximum (100%) yield of diamond was observed for mixtures with F/H ratio equal to 1:7. It should be noted that the observed temperature threshold (900 °C) for formation of diamond from binary mixture of CF1.1 and naphthalene is significantly lower than the initial temperature of diamond formation from naphthalene and other aromatic hydrocarbons, where it was found to be ∼1300 °C at 8 GPa. Figure 8 shows SEM images of diamonds produced by treatment of homogeneous mixtures of CF1.1 with naphthalene at 1:4 mass ratio under pressure of 8 GPa and temperature of 1000 °C. According to these images, the diamond fraction mainly consists of well-faceted crystals predominantly shaped into octahedra 21006

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The Journal of Physical Chemistry C with linear sizes in the range of 0.5 15 μm (Figure 8a). The formation of diamonds of this type is characteristic for processes of high-pressure thermal transformations of naphthalene and other hydrocarbons.8 However, the distinctive feature of transformation of binary mixture of CF1.1 and naphthalene is that in this case along with bulk microscale-size crystals the nanosize diamond fraction is also formed, as shown in Figure 8b,c. This fraction looks on the SEM image like a uniform mass of discrete particles sized in the 10 60 nm range and lacking a well-defined crystalline form. Nevertheless, it is interesting to note that according to TEM images shown in Figure 9 many of these diamond nanoparticles with linear sizes as small as 14 20 nm already demonstrate a clearly featured crystal faces in the form of octahedra, truncated octahedra, and twinned hexagonal plates.

4. CONCLUSIONS Therefore, the obtained results show that thermal transformations of homogeneous mixtures of CF1.1 with naphthalene under high pressures are characterized by substantial decrease in graphitization temperature of carbonization products obtained from both components of the mixture. Also, a substantial decline of initial temperature threshold for diamond synthesis as compared with pure hydrocarbon growth systems has been achieved. Given the fact that formation of highly ordered graphite in this case becomes a transformation step preceding the conception of diamond, it can be concluded that the mechanism of diamond formation in binary system involves nucleation of diamond on hydrogenated edges of graphite particles, followed by recrystallization of whole graphite particle into diamond, as proposed for diamond synthesis in hydrocarbon systems.17 Similarity of morphology and size of microcrystalline diamond fraction, produced at 8 GPa from binary mixtures of CF1.1 and naphthalene, to diamonds obtained from pure naphthalene provides additional confirmation for likeness of the mechanisms of micrometersize diamond formation in fluorine hydrogen-carbon and hydrocarbon systems. The observed reduction of diamond synthesis temperature in binary systems of CF1.1 and naphthalene in comparison with pure naphthalene can be explained by synergistic effect of fluorine hydrogen chemical interaction on carbonization processes enabling the graphitization of carbon residues to proceed at lower temperature. Taking into account the fact that simultaneous mass formation of nano- and micrometer-size diamond fractions has not been observed during high-pressure studies of thermal transformations of pure hydrocarbons, it could be hypothesized that in the case of binary mixtures formation of nanosize fraction of diamond proceeds through interlayer cross-linking polymerization of folded carbon layers during the process of carbonization of CF1.1 in hydrogen-containing media. However, direct evidence of realization of such mechanism of diamond formation has not been obtained in this work. Even considering that micrometer-size fraction of diamond represents a typical product of transformation of naphthalene and taking into account a homogeneous character of starting mixture prepared by thorough grinding of CF1.1 and naphthalene powders, it cannot be unambiguously stated that micrometer-size diamond fraction is formed from carbon residue of carbonization of naphthalene, whereas the nanosize fraction is produced from product of CF1.1 degradation. At the present, it can only be assumed that formation of large diamond crystallites as major products of transformation of binary mixtures may occur through phase transformation of

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large crystallites of graphite, according to mechanism proposed in work,17 whereas the formation of nanosize diamond fraction may follow a somewhat different mechanism. We hope that a more definite view on the mechanism of high-pressure diamond synthesis in binary systems will be created in the course of continuing studies using a coarse inhomogeneous mixtures of CF1.1 with naphthalene. This will help to reveal morphology peculiarities of diamonds formed from different components of starting mixture in a common fluorine hydrogen environment.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the Russian Fund of Basic Research (RFBR, grant no. 09-03-00752) and partially by award no. RUE22894-TI-07 of the U.S. Civilian Research & Development Foundation for the Independent States of the Former Soviet Union (CRDF). ’ REFERENCES (1) Wentorf, R. H. J. Phys. Chem. 1965, 69, 3063–3069. (2) Marsh, H.; Dachille, F.; Melvin, J.; Walker, J. P. L. Carbon 1971, 9, 159–177. (3) Whang, P. W.; Dachille, F.; Walker, J. P. L. High Temp. - High Pressures 1974, 6, 127–136. (4) Voronov, O. A.; Gavrilov, V. V.; Zhulin, V. M.; Rakhmanina, A. V.; Khlybov, E. P.; Yakovlev, E. N. Dokl. Akad. Nauk 1984, 274, 100–102. (5) Voronov, O. A.; Rakhmanina, A. V. Inorg. Mater. 1992, 28, 1408– 1413. (6) Yakovlev, E. N.; Voronov, O. A. Diamonds Superhard Mater. 1982, 7, 1–2. (7) Benedetti, L. R.; Nguyen, J. H.; Caldwell, W. A.; Liu, H.; Kruger, M.; Jeanloz, R. Science 1999, 286, 100–102. (8) Davydov, V. A.; Rakhmanina, A. V.; Agafonov, V.; Narymbetov, B.; Boudou, J. P.; Szwarc, H. Carbon 2004, 42, 261–269. (9) Davydov, V. A.; Rakhmanina, A. V.; Boudou, J. P.; Thorel, A.; Allouchi, H.; Agafonov, V. Carbon 2006, 44, 2015–2020. (10) Jennings, E.; Montgomery, W.; Lerch, P. J. Phys. Chem. B 2010, 114, 15753–15758. (11) Noda, T.; Kato, H. Carbon 1965, 3, 289–297. (12) Voronov, O. A.; Kasatochkin, A. V.; Radimov, N. P.; Kostikov, V. I.; Rakhmanina, A. B.; Khlybov, E. P. Inorg. Mater. 1983, 19, 1994–1996. (13) Onodera, A.; Terashima, K.; Urushihara, T.; Suito, K.; Sumiya, H.; Satoh, S. J. Mater. Sci. 1997, 32, 4309–4318. (14) Niedbalska, A. High Pressure Res. 1990, 5, 708–710. (15) Bundy, F. P. J. Chem. Phys. 1963, 38, 631–643. (16) Vereschagin, L. F.; Ryabinin, O. N.; Semerchan, A. A.; Lifshits, L. D.; Demyashkevich, B. P.; Popova, S. B. Dokl. Akad. Nauk 1972, 206, 78–79. (17) Lambrecht, W. R. L.; Lee, C. H.; Segall, B.; Angus, J. C.; Li, Z.; Sunkara, M. Nature 1993, 364, 607–610. (18) Das, D.; Singh, R. N. Int. Mater. Rev. 2007, 52, 29–64. (19) Butler, J. E.; Mankelevich, Y. A.; Cheesman, A.; Ma, J.; Ashfold, M. N. R. J. Phys.: Condens. Matter 2009, 21, 1–20. (20) Richley, J. C.; Harvey, J. N.; Ashfold, M. N. R. J. Phys. Chem. A 2009, 113, 11416–11422. (21) Pierson, H. O. Handbook of Carbon, Graphite, Diamond and Fullerenes; Noyes Publications: Park Ridge, NJ, 1993. 21007

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