Phase diagram

An interesting phase diagram is the diagram of carbon as can be seen below or in this article.  Carbon, at standard temperature and pressure, is in the form graphite.  This solid state is crystalline hexagonal shape.  Another common form of carbon is diamond.  As seen in the diagram, diamond only forms at extremely high pressures thus the solid form of carbon has a more compact structure and is tetrahedral.  The article also says that there is evidence that another solid phase that is metallic and cubic in structure exists at extreme temperatures and pressures.

In the diagram, there are several intermediate phases that occur in between graphite and diamond. These phases are actually consisting of two types of structures; a metastable diamond form with graphite at lower pressures (carbyne), and the metastable graphite with diamond at higher pressures (chaoite). The triple point of the graphite, diamond, and liquid phases is at about 4300-4700K and 10 GPa which is also where coexistence lines for the metastable forms intersect.  Therefore, if the authors are correct, there are 5 phases in equilibrium at this point.  The second triple point for liquid, graphite and vapor occurs at almost the same temperature, but at much lower pressures.

Also, in viewing the diagram, one can see that graphite will sublime to form the vapor form of carbon at standard pressure and elevated temperatures.  The liquid form on the other hand requires both higher temperatures and pressures (at least 4×103K and about .2 GPa).  Another characteristic of carbon is that the appearance of the gas phase at all temperatures ends rather abruptly with increase in pressure.

Carbon is strange in the fact that all of these transitions occur at such high temperatures and pressures, but other than that, it’s just another element.


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