Pressure-temperature phase diagram of elemental carbon

Carbon atoms form very strong bonds to each other yielding solid crystalline materials like graphite and diamond. Because of the high bonding energies, the vaporization and melting temperatures are very high. Different kinds of atom-to-atom bonding make many solid forms possible, ranging from pure graphite to pure diamond, as well as many types of molecules in liquid or gaseous carbon. Rigorous conditions of high temperature, high pressure, or both, are required to change a given elemental phase of carbon to another. Currently the vapor-pressure line of graphite, the P, T equilibrium line between graphite and diamond, and the graphite/diamond/liquid triple point are fairly well established. The triple point (or points) for graphite (or carbynes)/vapor/liquid remain controversial. At pressures less than 0.1 GPa liquid carbon seems to be a poor electric conductor while at higher pressures it is a good one. Current experimental and theoretical evidence indicate that diamond is stable against collapse to metallic forms (unlike Si and Ge) up to pressures over 350 GPa, and possibly as high as 2300 GPa. The latest static and shock compression experiments on diamond indicate that it melts to a conducting liquid at about 5000 K at pressures of 15 to 30 GPa, but does not melt at about 6000 K at 125 GPa. This suggests that the melting temperature of diamond increases with pressure, and that at the melting temperature liquid carbon is slightly less dense than diamond.