Hexagonal diamond: Scientists create bulk samples of ultra-hard carbon allotrope predicted practically 60 years in the past

The first bulk synthesis of hexagonal diamond marks a milestone for carbon allotropes, providing researchers a chance to extensively characterise this distinctive materials.

Diamond is without doubt one of the hardest supplies identified to exist in nature, arising from its construction during which carbon atoms covalently bond collectively in an ideal tetrahedral association. Nearly 60 years in the past, scientists predicted a more durable, various type often known as hexagonal diamond, which has a hexagonal lattice, reasonably than the cubic lattice adopted by typical diamond.

Natural hexagonal diamond has been found on Earth and is assumed to have shaped throughout meteorite strikes when the immense temperatures and pressures can quickly remodel graphite into this uncommon type of diamond. However, solely small grains of this pure hexagonal diamond have ever been found, blended in with cubic diamond and graphite. Methods replicating the warmth and stress of a meteorite strike within the lab have usually resulted in nanocrystalline constructions of hexagonal diamond, with these samples usually being impure, making it tough to review hexagonal diamond in isolation.

‘Now we have made a millimetre-sized chunk of [near] pure hexagonal diamond,’ says Ho-Kwang Mao on the Centre for High Pressure Science and Technology Advanced Research in China.

To do that, Mao and his staff within the US and China utilized about 200,000 instances atmospheric stress to a single crystal of pure graphite utilizing a diamond anvil cell. In situ x-ray diffraction earlier than and after making use of this stress allowed the scientists to watch the microscopic conversion of graphite to hexagonal diamond. With the pattern nonetheless beneath stress, laser heating at 1400°C stabilised the section, permitting the researchers to recuperate and subsequently research the near-pure pattern.

‘This new two-step method provides the first definitive evidence of hexagonal diamond as a distinct and recoverable bulk material,’ says Eiichi Nakamura, an inorganic chemist on the University of Tokyo who has beforehand labored on carbon allotropes.

Through this methodology, the staff synthesised crystals of close to pure hexagonal diamond – containing just a few microcrystals of the extra acquainted cubic diamond. The crystals ranged from 100μm to a number of millimetres in dimension, marking a primary for forming a definite and recoverable quantity of the fabric.

‘If you make [the bonding pattern of carbon atoms] three dimensional, there are only two ways of packing the layers,’ says Mao. ‘There’s ABC packing inside cubic diamond, and AB packing for hexagonal.’ High-resolution transmission electron microscopy confirmed that the pattern had AB stacking of buckled honeycomb layers, a construction indicative of hexagonal diamond.

The scientists probed the construction additional utilizing varied spectroscopic strategies. ‘We found that one of the bonds between the layers is actually shorter, compared to the other three, so this helps explain why the structure is stronger [compared with cubic diamond],’ says Mao. Results additionally confirmed that every one bonds have been sp³ σ bonds, with no sp² π bonds that will sign the presence of graphite.

The staff examined the hardness of the fabric utilizing a 1mm diameter disc of hexagonal diamond, discovering that the hardness was corresponding to pure diamond, as a result of minor cubic diamond defects. Future efforts will probably concentrate on refining synthesis circumstances, with Nakamura noting that ‘this [synthetic] breakthrough marks a milestone in the study of carbon allotropes’.