Direct synthesis of millimeter-sized hexagonal diamond from graphite.

Diamond has the strongest three-dimensional network structure and its cubic configuration is extremely stable under high pressure, thus limiting the experimental synthesis of diamond polymorphs. Hexagonal diamond, a typical polymorph of diamond, has attracted considerable attention in recent decades, yet synthesizing pure and large-sized hexagonal diamond remains technically challenging, preventing an accurate understanding of its properties and formation mechanism. Here, we report the direct synthesis of millimeter-sized, nearly pure hexagonal diamond from graphite under high-pressure and high-temperature conditions using our developed high-pressure technique in a multi-anvil press. The synthesized hexagonal diamond is highly oriented polycrystalline, exhibiting an ultrahard hardness (165 ± 4 GPa) on (100) planes, which is ∼50% harder than single-crystal cubic diamond. Structural characterizations and molecular dynamics simulations indicate that hexagonal diamond is formed through a martensitic transformation process whereby hexagonal graphite is transformed into hexagonal diamond by sliding and then direct bonding between graphite sheets. Furthermore, we show that the transformations from graphite to cubic or hexagonal diamonds are strongly temperature-pressure dependent. With this understanding, we further synthesized cubic/hexagonal diamond composites with unusual heterostructures at a lower pressure. This work not only established a fundamental framework for high-pressure phase transformations in graphite but also provided insight into the structural evolution of two-dimensional materials at high pressures and a potent strategy for exploring their new high-pressure phases.