Direct synthesis of millimeter-sized hexagonal diamond from graphite

Area of Science:

• Materials Science
• Geophysics
• Solid State Physics

Background:

• Diamond's cubic structure is stable under high pressure, limiting synthesis of its polymorphs.
• Hexagonal diamond, a polymorph, is difficult to synthesize in pure, large forms, hindering property and mechanism studies.

Purpose of the Study:

• To report the direct synthesis of millimeter-sized, nearly pure hexagonal diamond.
• To investigate the formation mechanism and properties of synthesized hexagonal diamond.
• To explore graphite's high-pressure phase transformations.

Main Methods:

• Developed a high-pressure technique using a multi-anvil press.
• Synthesized hexagonal diamond from graphite under high-pressure and high-temperature conditions.
• Employed structural characterizations and molecular dynamics simulations.

Main Results:

• Achieved direct synthesis of millimeter-sized, nearly pure hexagonal diamond.
• Synthesized hexagonal diamond exhibits ultrahardness (165 ± 4 GPa), exceeding single-crystal cubic diamond by ~50%.
• Identified a martensitic transformation process involving graphite sheet sliding and bonding for hexagonal diamond formation.
• Demonstrated temperature-pressure dependence of graphite to cubic/hexagonal diamond transformations.
• Synthesized cubic/hexagonal diamond composites with novel heterostructures at lower pressures.

Conclusions:

• Established a framework for high-pressure phase transformations in graphite.
• Provided insights into the structural evolution of 2D materials under high pressure.
• Developed a strategy for exploring new high-pressure phases of materials.