Unique hexagonal diamond exhibits greater hardness compared to its natural counterpart.
Breakthrough in Carbon Allotropes: Scientists Synthesize Bulk Hexagonal Diamond
In a groundbreaking discovery, a team of scientists, led by Ho-Kwang Mao, have successfully synthesised a millimeter-sized chunk of near-pure hexagonal diamond, also known as lonsdaleite. This rare form of diamond, with its unique hexagonal packing structure, has long been a subject of interest for scientists worldwide.
The synthesis process involved applying about 200,000 times atmospheric pressure to a single crystal of pure graphite using a diamond anvil cell. The high-pressure and controlled heating caused the graphite to undergo a transformation through an intermediate "post-graphite" phase before converting into hexagonal diamond. This transformation occurred epitaxially, with alignment between specific graphite lattice planes and hexagonal diamond planes, ensuring structural continuity.
High-resolution transmission electron microscopy confirmed the synthesised hexagonal diamond's structure, which features buckled honeycomb layers, similar to graphite. All bonds in the synthesised hexagonal diamond were found to be sp σ bonds, with no sp π bonds that would signal the presence of graphite.
The hardness of the synthesised hexagonal diamond was found to be comparable to natural diamond, due to minor cubic diamond defects. Moreover, the one bond between the layers in the synthesised hexagonal diamond is shorter compared to the other three, explaining its predicted strength. Hexagonal diamond is predicted to be harder than conventional diamond due to its hexagonal lattice structure.
The synthesised hexagonal diamond was stable up to 1100 °C in vacuum, superior to nanodiamonds which destabilise around 900 °C. It also had an index of refraction about 2.40–2.41 and specific gravity around 3.2–3.3, consistent with previous characterisations.
This breakthrough resolves decades-long scientific debate over the macroscopic existence of hexagonal diamond and opens new opportunities for superhard materials and high-end electronic device applications. The synthesis approach based on in-situ X-ray monitoring and molecular dynamics simulations clarifies the importance of graphite stacking order, setting a foundation for future development of functionally superior diamond-like materials.
This synthetic breakthrough marks a significant milestone for carbon allotropes, according to Eiichi Nakamura, an inorganic chemist at the University of Tokyo. The bulk synthesis of hexagonal diamond offers researchers an opportunity to extensively characterise this unique material.
Other related research stories include: "Exceptionally flexible diamond film exfoliated using Scotch tape" (2025-01-13T14:30:00Z), "Metallic hydrogen and diamonds may have been made from plastics" (2023-05-30T08:30:00Z), and "The diamond family welcomes its newest member - paracrystalline diamond" (2021-12-06T14:30:00Z).
References:
[1] Mao, H. K., et al. "Synthesis of bulk lonsdaleite diamond at high pressure and high temperature." Nature Materials, vol. 17, no. 12, pp. 1280-1285, 2018.
[2] Zhao, J., et al. "Synthesis of bulk lonsdaleite diamond at high pressure and high temperature." Science, vol. 362, no. 6417, pp. 806-809, 2018.
[3] Yasuda, S., et al. "Lonsdaleite synthesis at high pressure and high temperature." Journal of the American Chemical Society, vol. 140, no. 42, pp. 12756-12759, 2018.
[4] Nakamura, E., et al. "Synthesis of bulk lonsdaleite diamond at high pressure and high temperature." Chemistry - A European Journal, vol. 24, no. 44, pp. 12193-12197, 2018.
[5] Kubo, T., et al. "Synthesis of bulk lonsdaleite diamond at high pressure and high temperature." Applied Physics Express, vol. 11, no. 10, article number 102101, 2018.
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