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Why Does A Graphite Turn Into Hexagonal Diamond, Not Cubic?

First Posted: Feb 24, 2017 03:28 AM EST
Graphite Converting To Diamond
When exposed to high temperatures, graphite turns into a hexagonal diamond rather than the usual cubic diamond.
(Photo : Bart Klarenaar/YouTube screenshot)

The scientists have been trying to figure out for years why graphite turns into hexagonal diamond when exposed to high pressure and not the usual cubic diamond. Now, they finally resolve it and find answers.

The study was published in the recent issue of the Journal of the American Chemical Society. It was led by Xiao-Jie Zhang, Yao-Ping Xie and Zhi-Pan Liu from Fudan University and Shanghai University in Shanghai, China.

Liu said their study resolves the long-standing puzzle of why hexagonal diamond is specially created from graphite instead of the cubic diamond at the onset of the diamond formation. He further said that considering that graphite-to-diamond is a prototype solid-to-solid transition, the knowledge learned from this study should greatly benefit the understanding of high-pressure solid physics and chemistry.

The reason for the conversion of graphite into hexagonal diamond, not cubic, is due to speed or referred to as reaction kinetics in chemistry. The researchers used a brand-new type of simulation, in which they determined the lowest-energy pathways in the graphite-to-diamond transition. They discovered that the transition to hexagonal diamond is about 40 times faster than the transition of the cubic diamond. This means that even though the cubic does begin to shape, there is a large amount of hexagonal diamond that mixed in, according to Phys.org.

Graphite is a form of crystalline carbon that is found in metamorphic and igneous rocks. It is a mineral that is particularly soft and has a very low specific gravity. It also splits with very light pressure and resistant to heat. Graphite is often used in manufacturing and metallurgy.

Meanwhile, the hexagonal and cubic diamonds comprise of carbon atoms with maximum four bonds. Therefore, the diamond is hard. The cubic diamond has layers that are all oriented in the same direction. On the other hand, the hexagonal diamond, the layers are alternately oriented, which resulted to hexagonal symmetry.

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