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Metastable Nanosized Diamond Formation from Fluid Systems S. K. Simakov^a, ^aGeological Department, St.Petersburg University, 7/9 Universitetskaya Nab., St.Petersburg, 199034, Russia

The model of nanosized diamond particles formation at metastable P-T parameters from fluid is presented. It explains the specific of CVD diamond synthesis gases mixtures and hydrothermal growth of diamond at low P-T parameters as well as it explains the geneses of metamorphic and magmatic nano- and microdiamond in the shallow depth Earth rocks and the genesis of interstellar nanodiamond formations in the space.

The optimal gases system compositions for metastable diamond formations have been long debated in many publications during the long period. Badziag et al.¹ came to the conclusion that nanometer-sized diamonds could be more stable than graphite when formed from hydrocarbons with a H/C ratio of more than 0.24. Simakov² has shown that the addition of oxygen to the hydrocarbon gases can stabilize the diamond nucleus formation in the P-T field of graphite stability. The question of whether diamond could be grown under hydrothermal conditions mimicking those under which it is formed in the Earth has been also long debated^3,4. Roy et al.⁵ have made the detailed study of the hydrothermal growth of diamond in the C-H-O and C-H-O-halogen systems. Shimansky et al.⁶ have claimed hydrothermal growth of diamond but no details were given on the composition of the liquid and the characterization of the phases. Zhao et al.⁷ provided the diamond hydrothermal synthesis from the mixture of the glassy carbon, powdered nickel, diamond seeds and water at 800^o C and 1.4 kbar. From another hand, Bachmann et al.⁸ have analysed gaseous compositions of the different 78 CVD diamond syntheses and shown that their compositions lie in the corridor extended range from H to CO compositions of the C-O-H system. The triangularly shaped “diamond domain” is adjusted along the CO line of the diagram and is limited by the lines X_CO2 ≈ 0.57 on its carbon-rich side and X_CO2 ≈ 0.45 on its oxygen-rich side (see Fig. 1 in ref. 8). The main part of the synthesis compositions lies in the middle part of the domain between methane and acetylene lines meanwhile only the lesser part corresponds to hydrogen.

For diamond, it has been argued, that crystallization under P-T conditions, where diamond is actually thermodynamically unstable with respect to graphite, is possible due to kinetic factors ^9,10. Nanosize diamond particles have energetic preference upon graphitic particles of the same size and could be more stable at low P-T parameters (ref.1)^11-13. Fedoseev et al.¹⁴ have shown that critical radii of graphite and diamond nuclei depends upon the surface energy (s), atomic volume of carbon (V) and chemical potential of the resaturation (Dm):

r_g/r_d = V_gs_g Dm_d / V_ds_d Dm_g (1)

s of nanosized diamond (d) and graphite (g) particles depends upon temperature and size of the particles¹⁵. From s_g/s_d relation given in¹⁶ and from (ref.12) it follows that for nanoparticles V_gs_g/V_ds_d ≈ 1. Than r_g/r_d ratio could be expressed as:

r_g/r_d = Dm_d / Dm_g (2)

On the other hand the chemical potentials of the resaturation for diamond and graphite could be expressed as follows:

Dm_g =RTln(Pi/Pi_g) (3)

Dm_d =RTln(Pi/Pi_d) (4)

where Pi and Pi_g,d are the real and equilibrium pressures of carbonaceous gases

As it follows from equations (2-4), (r_g/r_d) depends on the ratio between real and calculated equilibrium pressures of carbonaceous gases. Within the range of graphite stability Pi_g <>d, which corresponds to preferable graphite formation from fluid phase. The condition of preferable diamond formation corresponds to Pi_g > Pi_d. The difference between Dm_g and Dm_d depends upon the difference of Pi/Pi_g and Pi/Pi_d , and at lower Pi_car it tends to zero, which corresponds to optimal condition of diamond formation from gaseous mixture within the range of graphite stability.

In the hydrocarbon-hydrogen mixture the gas-solid reaction of hydrocarbon destruction could be proposed for carbon formation in a fluid:

CH₄ ® C + 2H₂ (I)

At high temperatures equilibrated pressure of hydrogen would be greater than equilibrated pressure of methane and in vacuum P_CH4(d) tends to P_CH4(g) (Fig.1,A). The predomination of hydrogen under hydrocarbon in the gaseous mixture has also suppressed the growth rate of graphite more than it suppressed the growth rate of diamond (ref.10). As a result, it could stabilize the diamond growth at these conditions. Based on this effect, Deryagin and Fedoseev (ref.9) have grown diamond on the diamond seeds at vacuum conditions.

Subsequent work has shown that the addition of oxygen to the hydrocarbon gases can stabilize the diamond nucleus formation in the P-T range of graphite stability (ref.2). This conclusion coincides with the established fact that diamond is more stable in the oxygen environment than graphite, because oxygen reduces graphite to a greater degree than diamond¹⁷. Calculations done for the C-O-H system show that P_CH4(car) is very low within the ranges of the system, close to the upper limit of carbon stability by oxygen (CCO buffer)¹⁸. Within the ranges at lower pressure and temperature, P_CH4(g) ≈ P_CH4(d), which corresponds to diamond nucleus stabilization.

The presented model explains the extended Bachmann’s “diamond domain” along the CO line from H to CO compositions in the C-O-H system (ref. 8). The fluid calculations performed at 1000^o C and 10^-3 bar within the wide range of oxygen fugacity show that the diamond stability range corresponds in more degree to CO composition and in less degree to H₂ composition of the gases mixture (Fig. 1,A).

From the calculations it follows that this model provides a common basis for low-pressure diamond CVD methods. It comprises and connects data for more than 30 years of diamond CVD. By means of this model, special relations between very different source gasses and gas mixtures become clear. On the other hand, the model explains the possibility of metastable hydrothermal growth of the diamond too (Fig. 1, C). Our experiments at 500°C and total pressure of nearly 1000 bar from water liquid of organic matter proved the possibility of nanodiamond formation from C-O-H fluids at low temperatures and pressures without seeds ¹⁹. The determined relations may help to develop new models of the surface processes and growth species needed for diamond deposition.

It is known that the bulk of Earth diamonds is formed due to the deep upper mantle rocks - kimberlites formed at P and T corresponding to diamond thermodynamic stability. Meanwhile for the last 40 years micro- and nanodiamonds have also been found in shallow metamorphic earth rocks formed at P-T parameters corresponding to graphite thermodynamic stability ^20-22 as well as in the basalts ^23,24. The highest grade is observed in hydrothermal metasomatic zones of Kokchetave metamorphic massive situated in Northern Kazakhstan ²⁵. The fluid calculations performed at P-T parameters corresponded to Hawaiian basalt formation show that the diamond stability range here corresponds to CO₂ and H₂O compositions of the fluid (Fig. 1,B). It explains the relationship of Hawaiian nanodiamonds with carbon dioxide fluids (ref.24). The calculations performed at lower P-T parameters corresponded to hydrothermal metasomatic zones of Kokchetave metamorphic massive formation show that the diamond stability range also corresponds to CO₂ and H₂O compositions of the fluid (Fig. 1,C). It explains the relationship of the Kokchetave diamonds with water and carbon dioxide²⁶.

The origin of diamonds in the interstellar space has been a topic of intense discussion since the discovery of presolar nanodiamonds in chondrites ²⁷. Meteoritic nanodiamonds provide information on the nucleosynthesis of evolved stars and the evolution of the astrophysical environment, which formed the solar system. Sellgren ²⁸ identified the relationship between the interstellar diamond and water ice. Nakano et al.²⁹ related interstellar diamond formation with organic matter. Based on these relationships Kouch et al.³⁰ identified new formation routes of diamond in the interstellar clouds and parent bodies of carbonaceous chondrites during laboratory experiments. It’s the ice mixture of H₂O, CO, NH₃ and CH ₄ (4 : 2 : 2 : 1). The questions of when and how does nanodiamonds originate in the Cosmos remain open, although comparative microstructural analysis of nanodiamonds extracted from meteorites, indicates that the majority of cosmic nanodiamonds are formed by low-pressure vapor condensation³¹. The fluid calculations performed at 250^o C and 10^-3 bar show that the diamond stability range here corresponds to CO₂ and H₂O compositions of the fluid (Fig. 1,D). It explains the relationship of interstellar diamonds with water (ref. 28).

The presented in the paper model explains the specificity of the CVD diamond synthesis gas compositions and the hydrothermal growth of diamond at low P-T parameters as well as the geneses of metamorphic and magmatic nano- and microdiamond in the shallow depth Earth rocks and of interstellar nanodiamonds in the space at P-T parameters corresponding to graphite stability. Nanosized diamond particles could be formed from carbon-bearing fluids at low temperatures and pressures without seeds in the range of the upper limit of carbon stability by oxygen.

References:

1. Badziag, P., Verwoerd, W.S., Ellis, W.P., Greimer N.R. Nanometer-sized diamonds are more stable than graphite. Nature. 343, 244-245. (1990)

2. Simakov, S.K. Thermodynamic estimation of oxygen-hydrogen conditions influence on diamond and graphite critical nucleus formation at processes of methane destruction at low pressures. Rus. J. Phys.-Chem. 69, 3460-347. (1995)

3. Shatsky, V. S., Sobolev, N. V. Origin of diamonds in metamorphic rocks. Dokl. Akad. Nauk. 331, 217-219. (1993)

4. DeVries, R. C., Roy, R., Somiya, S., Yamada, S. A review of liquid phase systems pertinent to diamond synthesis. Trans. Mat. Res. Soc. Jap. 14B, 1421-1445. (1994)

5. Roy, R., Ravichandran, D., Ravindranathan, P., Badzian, A. Evidence for hydrothermal growth of diamond in the C-H-O and C-H-O halogen system. J. Materi. Res., 11, 1164-1168. (1996)

6. Szymanski, A., Abgarowicz, E., Bakon, A., Niedbalska, A., Salacinski, R., Sentek, J. Diamond formed at low pressures and temperatures through liquid-phase hydrothermal synthesis. Diam. Relat. Mater., 4, 234-235. (1995)

7. Xing-Zhong Zhao, Rustum, R., Kuruvilla A. C., A. Badzian. Hydrothermal growth of diamond in metal–C–H₂O systems. Nature 385, 513 – 515. (1996)

8. Bachmann, P.K., Leers, D., Lydtin, H. Towards a general concept of diamond chemical vapour deposition. Diam. Relat. Mater., 1, 1-12. (1991)

9. Deryagin, B.V. & Fedoseev, D.V. Growth of diamond and graphite from the gas phase. Nauka, Moscow. 115 p. (1977)

10. Chauhan, S.P., Angus, J.C. & Gardner, N.C. Kinetics of carbon deposition on diamond powder. J.Appl. Phys. 47, 4746-4754 (1976)

11. Chaikovskii, E.F., Rosenberg, G.H. Phase diagram of carbon and possibility of diamond formation at low pressures. Dokl. Akad. Nauk SSSR 279, 1372-1375. (1984)

12. Gamarnik, M.Y. Energetical preference of diamond nanoparticles. Physical Rev. B. 54, 2150-2156. (1996)

13. Tawson, V.L., Abramovich, M.G. Polymorphism of crystals and phases size effect: transformation diamond to graphite. Dokl. Akad. Nauk SSSR 287, 291-295. (1986)

14. Fedoseev, D.V., Deryagin, B.V., Varshavskaya, I.G., Semenova-Tyan-Shanskaya, A.S. Diamond crystallization. Nauka, Moscow. 134 p. (1984)

15. Magomedov, M.N. About the relationship of surface energy with size and form of nanocrystals. Phys. Tverd. Tela. 46, 924-937. (2004)

16. Nuth, J. A. Small-particle physics and interstellar diamonds. Nature. 329, 589. (1987)

17. Rudenko, A.P., Kulakova, I.I., Skvortsova, V.I. Chemical diamond synthesis. Aspects of general theory. Usp. Him. (Rus. Chem. Rev.) 62, 99-117. (1993)

18. Simakov, S.K. Redox state of Earth's upper mantle peridotites under the ancient cratons and its connection with diamond genesis. Geoch. Cosm. Acta. 62, 1811-1820. (1998)

19. Simakov, S.K., Dubinchuk, V.T., Baidakova, M.V. Synthesis of nanosize diamond and diamondlike phases at lower temperatures and pressures. NDNC2007 Abstract Book. 279. (2007)

20. Rozen, O.M., Zorin, U.M. & Zayachkovsky, A.A. Diamond foundation in connection of precambrian eclogites of Kokchetave massive. Dokl. Akad. Nauk SSSR. 203, 674-676. (1972)

21. Dobrzhinetskaya, L.F., Eide, E.A., Larsen, R.B., Sturt, B.A., Tronnes, R.G., Smith, D.C., Taylor, W.R., Posukhova, T.V. Microdiamonds in high-grade metamorphic rocks of the Western Gneiss region, Norway. Geology. 23, 597-600. (1995)

22. Sobolev, N.V., Shatsky, V.S. Diamond inclusions in garnets from metamorphic rocks; a new environment for diamond formation. Nature. 343, 742-746. (1990)

23. Novgorodova, M.I., Rasskazov, A.V. High-pressure carbon mineral phase formation as a result of heat explosion at shift transformation of graphite. Dokl. Akad. Nauk SSSR. 322, 379-381. (1992)

24. Wirth, R., Rocholl, A. Nanocrystalline diamonds from the Earth’s mantle underneath Hawaii. Earth Plan. Scie. Lett. 211, 357-362. (2003)

25. Pechnikov, V.A., Kaminsky, F.V. Diamond potential of metamorphic rocks in the Kokchetav Massif, northern Kazakhstan. Eur. J. Mineral. 20, 395–413. (2008)

26. De Corte, K., Cartigny, P., Shatsky, V.S., De Paepe, P., Sobolev, M.V., Jovay, M. Characteristics of microdiamond from UHPM rocks of the Kokchetav massif (Kazakhstan). Proc. VII^th Int. Kimb. Conf. 2, 174-182. (1999)

27. Bernatowicz, T., Zinner, E.. Astrophysical Implications of the Laboratory Study of Presolar Materials, AIP Conference Proceedings 402, New York. (1997)

28. Sellgren, K. Aromatic hydrocarbons, diamonds, and fullerence in interstellar space: puzzles to be solved by laboratory and theoretical astrochemistry. Spectrochimica Acta P.A. 57, 627-642. (2001)

29. Nakano, H., Kouchi, A., Arakawa, M., Kimura, Y., Kaito, C., Ohno, H., Hondoh, T. Alteration of interstellar organic materials in meteorites’ parent bodies: a novel route in diamond formation. Proc. Japan Acad. Ser. B. 78, 277-281. (2002)

30. Kouchi, A., Nakano, H., Kimura1, Y., Kaito, C. Novel routes for diamond formation in interstellar ices and meteoritic parent bodies. The Astrophysical Journal. 626, L129–L132. (2005)

31.Daulton, T.L. Extraterrestrial nanodiamonds in the cosmos. In: Ultrananocrystalline diamond. William-Andrew. Norwich. UK., 23-79 (2006)

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