Carbon atoms form very strong bonds to each other, yielding materials like: (i) crystalline graphite, diamond and their many “amorphous” hybrids; (ii) crystalline forms of giant closed–surface molecules such as the fullerenes; and (iii) liquid and gas phases which have molecular contents which are complicated and not yet defined or understood. Because of the high bonding energy the melting and vaporization temperatures of the solid forms are very high, and the activation energies required to transform one solid form to another are large. One consequence is that at lower temperatures the different solid phases may continue to exist metastably far into a P, T region in which another solid phase is the thermodynamically stable one.
In the thermodynamic sense the vapor pressure line of graphite, the graphite/liquid/vapor triple point, the graphite melting line, the graphite/diamond equilibrium line, and the graphite/diamond/liquid triple point are quite well established. Data for the melting temperature of diamond vs. pressure are sparse and rough, but they indicate that the melting temperature increases with pressure,-in agreement with some theories. Although carbon should transform to a solid metallic state at very high pressures, experimental evidence shows diamond to be stable to over 400GPa, and theoretical calculations indicate that it could be the stable form up to pressures of 1200 to 2300GPa. Attention is given to the solid state transformations which can take place when graphite is compressed and heated along different P, T paths under different conditions.