Posts Tagged ‘Tetragonal crystal system’

The never ending wonders of Carbon

January 27, 2011

Not just all life as we know it and coal and diamonds and graphite and carbon nanotubes and now the new wonder-world of  graphene.

Carbon also has the highest melting and sublimation point of all elements. At atmospheric pressure it has no melting point as its triple point is at 10.8 ± 0.2 MPa and 4600 ± 300 K, so it sublimates at about 3900 K.

File:Carbon basic phase diagram.png

Theoretical phase diagram of carbon: Wikipedia

Evidence is mounting that a new crystal form of carbon – body-centered tetragonal (bct) – something between diamond and graphene must exist. Simulations show that it must. It is now up to experimentalists to prove it.

Image: From "Ab Initio study of the formation of transparent carbon under pressure," by Xiang-Feng Zhou et al., in Physical Review B, Vol. 82, No. 13; October 29, 2010

From Scientific American:

Now evidence is mounting that there is yet another crystal structure to add to carbon’s catalogue of wonders: a material that could find applications in mechanical components whose hardness varies depending on the pressure to which they are exposed.

This new type of carbon was first observed in 2003, when researchers placed graphite, a stacking of chicken-wire-shaped networks of carbon atoms, under high pressure at room temperature. Under this “cold” compression, the graphite began to assume a hybrid form, between that of graphene and of diamond, but its exact nature was unknown.

Two computer simulation studies now suggest that cold-compressed graphite contains crystals of a structure called body-centered tetragonal, or bct, in addition to another type called M carbon. In bct, groups of four atoms are arranged in a square. The squares are stacked in an offset manner, and each square forms chemical bonds with four squares in the layers above and four below. A team led by Hui-Tian Wang of Nankai University in Tianjin, China, showed that during cold compression the transition to bct carbon results in a release of energy, which means it is likely to happen in the real world.

A Japanese and American team also conducted a simulation in which bct carbon produced x-ray patterns similar to those seen in the 2003 study. …. Whether bct carbon exists or can be synthesized in its pure form “is still a task for experimentalists to test.” 


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