Diamond, as the hardest material found in nature, is a critical foundational material for the development of strategic industries such as aerospace, national defense, and the third-generation semiconductor industry in China. Enhancing the hardness of diamond has always been a significant research topic in the field of superhard materials. Currently, it is widely believed that there are two main approaches to achieve this goal: (1) microstructure regulation and (2) synthesizing hexagonal diamond. However, how to construct microstructures such as nano-twins in diamond to improve its performance and how to synthesize hexagonal diamond with hardness exceeding that of cubic diamond remain unresolved challenges. To address these scientific issues, this report will detail the research progress made by our team in recent years on the phase transition mechanisms from graphite to cubic diamond and hexagonal diamond.

(1) Research on the Phase Transition from Graphite to Cubic Diamond

Through studies on the transformation of polycrystalline graphite under isotropic pressure conditions and the phase transition behavior of single-crystal graphite under stress in different directions (including in-plane stress greater than interlayer stress and vice versa), our group discovered that stress differences play a decisive role in the phase transition pathway. Furthermore, by examining the phase transition of "bent graphite" with different orientations (e.g., onion-like carbon) under hydrostatic pressure and high-temperature, high-pressure conditions, we found that in-plane stress induces graphite bending, thereby disrupting the symmetry of perfect graphite. This symmetry disruption determines the subsequent phase transition pathway, making it difficult to form hexagonal diamond. However, the bent graphite structure can induce the formation of nano-twinned diamond, significantly enhancing the material's performance.

(2) Research on the Phase Transition from Graphite to Hexagonal Diamond

Using non-equilibrium molecular dynamics simulations, our group further discovered that hexagonal diamond can only form when shock stress is applied along the [001] direction of graphite, while shocks in other directions disrupt the stacking structure of graphite. Displacement analysis revealed that under high-speed impact (>5.0 km/s), graphite undergoes interlayer collapse within an extremely short time (on the order of femtoseconds), forming interlayer C-C bonds. However, at this stage, graphite has not yet shifted to the displacement required for cubic diamond, resulting in the formation of metastable hexagonal diamond. Based on this finding, our team proposed a new strategy: applying quasi-uniaxial compression along the z-axis of graphite to preserve its intrinsic layered structure and prevent interlayer buckling and sliding. Simultaneously, by controlling a moderate temperature (1700-1800 K), we overcame the phase transition energy barrier while suppressing basal plane sliding. Using this method, our team, through large-scale molecular dynamics simulations, directly observed for the first time the "nucleation-growth" mechanism of the graphite-to-hexagonal diamond transition and experimentally validated the correctness of this phase transition mechanism.

This research provides new theoretical insights and technical pathways for enhancing diamond hardness and lays an important foundation for the controlled synthesis of hexagonal diamond.

Reference

[1] Zhu S., Yan X., Liu J., A. R. Oganov, Zhu Q., Matter, 2020, 3, 864.
[2] Chen G., Zhu S., Xu L., Li Y., Liu Z., Hou Y., and Mao H-k., JACS Au, 2024, 4, 3413.
[3] Zhu S., Chen G., Yuan X., Cheng Y., Wan M., Xu B., Wang M., Tang H., Hou Y., J. Am. Chem. Soc., 2025, 147, 2158.