Nanocrystalline hexagonal diamond formed from glassy carbon

Area of Science:

• Materials Science
• Solid State Physics
• Crystallography

Background:

• Carbon's diverse allotropes and phase behavior are areas of active research and uncertainty.
• Lonsdaleite, a hexagonal diamond polymorph, was previously discovered in meteorites and linked to impact events.
• Recent studies questioned lonsdaleite's existence, proposing it as defective cubic diamond formed under specific conditions.

Purpose of the Study:

• To synthesize high-purity lonsdaleite under controlled laboratory conditions.
• To investigate the formation mechanism of lonsdaleite and its relationship to strain.
• To challenge the exclusive link between lonsdaleite formation and extraterrestrial impacts.

Main Methods:

• Synthesis of lonsdaleite using a diamond anvil cell at 100 GPa and 400°C.
• Analysis of the synthesized nanocrystalline material via X-ray diffraction and high-resolution electron microscopy.
• Theoretical validation using first-principles calculations to model transformation pathways.

Main Results:

• Successful synthesis of almost pure lonsdaleite, a hexagonal diamond allotrope.
• Observation that intense radial plastic flow under compression facilitates the transformation.
• Identification of favorable stacking of graphene sheets as a key factor in lowering the energy barrier.
• Experimental evidence supporting strain-induced transformation of graphitic precursors to hexagonal diamond.

Conclusions:

• High-purity lonsdaleite can be readily synthesized under laboratory-induced strain.
• The formation of lonsdaleite is not exclusively dependent on meteorite impact conditions.
• Strain-induced transformation provides a plausible mechanism for lonsdaleite formation in specific geological or experimental settings.