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Metastable Nanosized Diamond Formation from Fluid Systems S. K. Simakova, 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 debated3,4. 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 800o 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 XCO2 0.57 on its carbon-rich side and XCO2 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):

rg/rd = Vgsg Dmd / Vdsd Dmg (1)

s of nanosized diamond (d) and graphite (g) particles depends upon temperature and size of the particles15. From sg/sd relation given in16 and from (ref.12) it follows that for nanoparticles Vgsg/Vdsd 1. Than rg/rd ratio could be expressed as:

rg/rd = Dmd / Dmg (2)

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

Dmg =RTln(Pi/Pig) (3)

Dmd =RTln(Pi/Pid) (4)

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

As it follows from equations (2-4), (rg/rd) depends on the ratio between real and calculated equilibrium pressures of carbonaceous gases. Within the range of graphite stability Pig <>d, which corresponds to preferable graphite formation from fluid phase. The condition of preferable diamond formation corresponds to Pig > Pid. The difference between Dmg and Dmd depends upon the difference of Pi/Pig and Pi/Pid , and at lower Picar 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 PCH4(d) tends to PCH4(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 PCH4(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, PCH4(g) PCH4(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 1000o 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 250o 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.


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Fig. 1. Variation of compositions of C-O-H system : A - at 1000o C and 10-3 bar, B - at 1000o C and 20 kbar, C - at 500o C and 1 kbar, D - at 250o C and 10-3 bar. CCO- upper limit of carbon by oxygen, point line – the boundary of the range of diamond preferable growth.





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