Graphite-diamond equilibrium

Graphite compounds, 12 777-778 Graphite crystal lattice, 17 46 Graphite crystals, 12 714, 776 Graphite-diamond equilibrium line, 8 531-532... 

The graphite-diamond equilibrium line up to 1200 K was calculated in 1938 (6) by using the observed heat, compressibility, and thermal expansion data of the two components (Fig. 1). Subsequendy, estimates of the diamond—graphite equilibrium line were refined and extended (9) and the extrapolation to higher temperatures fits the experimental data (10). It is evident that diamond is not thermodynamically stable below a pressure of about 1.6 GPa (16 kbar) and early investigators were using pressures in their experiments where diamond would have been unstable. 

Fig. 3. The graphite-diamond equilibrium curve. [After Berman and Simon (5).]...

Figure 1. P T diagram of the low-pressure, low-temperature labile equilibriums of carbon solution 1 = graphite-diamond equilibrium line, 2 = glassy carbon-diamond transition line, 3 = range of pneumatolytic hydrothermal processes, 4=oxidative corrosion of diamond, 5 = anticipated area of diamond hydrosynthesis, 6 and 7 = diamond synthesis from glassy carbon precursors, 8 = low-pressure, low-temperature hydrothermal homoepitaxy of diamond. Reproduced from [15] with permission from A. Szymanski.

The graphite-diamond equilibrium expression in this region, following Berman and Simon [14] is... 

The graphite-diamond equilibrium line in Fig. 7 is based on a thermodynamic ana-... 

Estimate the slope of the P-T curve for the graphite-diamond equilibrium at 25°C. At this temperature and... 

The (p,T) equilibrium line for the graphite-diamond transition is obtained from equation (15.14) by expressing AH° and AS° as a function of temperature and the integral term as a function ofpressure and temperature. The free energy change A Gpr is then set equal to zero (the equilibrium condition) and the resulting expression... 

In 1963 a phase diagram was established by Bundy and Wentorf [19], based on data of Wentorf [42] and experiments carried out at pressures higher than 4 GPa. This phase diagram described c-BN as the stable phase at standard temperature and pressure (Fig. 6). In 1975 a new phase diagram was published by Corrigan and Bundy [11], showing the c-BN/h-BN equilibrium line similar to the graphite/diamond line in the carbon system. 

O. C (5, diamond) C s, graphite) (This equilibrium can exist only under very special conditions.) Densities of diamond and graphite are 3.5 and 2.3g/cm , respectively. 

The density of diamond is 3.52 g/cm and that of graphite is 2.25 g/cm. At 25 °C the Gibbs energy of formation of diamond from graphite is 2.900 kJ/mol. At 25 °C what pressure must be applied to bring diamond and graphite into equilibrium ... 

The equilibrium diagram however includes a liquid-graphite-diamond triple point near 130 kbar and 4000 K. Along the equilibrium line between graphite and diamond at for instance 1800 K and 70 kbar diamond may also be formed under favorable circumstances. It is utilized at the modern syntheses of diamond, described in section 39.10. Perhaps this was also what happened in nature when the natural diamonds were created in gelogical processes deep in the earth. 

Figure 36, P-T phase and reaction diagram of carbon as results from Refs. 509 and 510. Solid lines represent equilibrium phase boundaries. The dashed line is the threshold for conversion of hexagonal diamond and both hexagonal and rhombohedral graphite into cubic diamond.

The main silicate inclusions in natural diamond are pyroxenes and garnet [12178-41 -5/, and the understanding of the conditions of their formation from laboratory studies is the basis for the determination of the P—T conditions when diamond was formed (2—6). CO, C02, H2, H O are also found in diamond (20), and it is possible that diamond nucleated and grew in a liquid in a C—H—O system, perhaps immiscible, but in equilibrium with the silicate matrix (4). Graphite [7440-44-0] is also a common inclusion in natural diamond. 

Crystal Morphology. Size, shape, color, and impurities are dependent on the conditions of synthesis (14—17). Lower temperatures favor dark colored, less pure crystals higher temperatures promote paler, purer crystals. Low pressures (5 GPa) and temperatures favor the development of cube faces, whereas higher pressures and temperatures produce octahedral faces. Nudeation and growth rates increase rapidly as the process pressure is raised above the diamond—graphite equilibrium pressure. 

Self-Test 9.16B Predict the effect on the equilibrium compositions of compressing a mixture in which diamond and graphite are in equilibrium. Consider the relative densities of the two solids, which are 3.5 g-cm 3 for diamond and 2.0 g-cm 3 for graphite. 

Since thermodynamics deals with systems at equilibrium, time is not a thermodynamic coordinate. One can calculate, for example, that if benzene(equilibrium with hydrogen(g) and carbon(s) at 298.15 K, then there would be very little benzene present since the equilibrium constant for the formation of benzene is 1.67 x 10-22. The equilibrium constant for the formation of diamond(s) from carbon(s, graphite) at 298.15 K is 0.310 that is, graphite is more stable than diamond. As a final example, the equilibrium constant for the following reaction at 298.15 K is 2.24 x 10-37 ... 

This equilibrium can only exist under very special conditions. Density of diamond = 3.5 density of graphite = 2.3 g/cm3. 

Diamond, graphite, and the fullerenes differ in their physical and chemical properties because of differences in the arrangement and bonding of the carbon atoms. Diamond is the densest (3.51 vs 2.22 and 1.72 g cm-3 for graphite and Cw, respectively), but graphite is more stable than diamond, by 2.9 kJ mol-1 at 300 K and 1 atm pressure it is considerably more stable than the fullerenes (see later). From the densities it follows that to transform graphite into diamond, pressure must be applied, and from the thermodynamic properties of the two allotropes it can be estimated that they would be in equilibrium at 300 K under a pressure of —15,000 atm. Of course, equilibrium is attained extremely slowly at this temperature, and this property allows the diamond structure to persist under ordinary conditions. 

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