Why does Graphite turn only into Hexagonal Diamond ?

 When exposed to moderately high pressures, why does graphite turn into hexagonal diamond (also called lonsdaleite) and not the more familiar cubic diamond, as predicted by theory?

Researchers have finally answered a question that has eluded scientists for years: The answer largely comes down to a matter of speed—or in chemistry terms, the reaction kinetics. Using a brand new type of simulation, the researchers identified the lowest-energy pathways in the graphite-to-diamond transition and found that the transition to hexagonal diamond is about 40 times faster than the transition to cubic diamond. Even when cubic diamond does begin to form, a large amount of hexagonal diamond is still mixed in.

Graphite, hexagonal diamond, and cubic diamond are all carbon allotropes, meaning they are made of carbon atoms that are arranged in different ways. Graphite consists of stacked layers of graphene, whose atoms are arranged in a honeycomb-like lattice. Since the carbon atoms in graphene are not fully bonded, graphene is soft and flakes easily, making it ideal for use as pencil lead.

Both types of diamond, on the other hand, consist of carbon atoms that all have the maximum four bonds, which explains why diamond is so hard. In cubic diamond (the kind typically found in jewelry), the layers are all oriented in the same direction. In hexagonal diamond, the layers are alternately oriented, giving it a hexagonal symmetry.

Under high pressures of more than 20 gigapascals (nearly 200,000 times atmospheric pressure), theory and experiment agree that graphite turns into cubic diamond, with some hexagonal diamond mixed in. But under pressures of less than 20 gigapascals, simulations have always predicted that cubic diamond should be the favored product, in contrast with experiments.

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