Abstract
The synthesis of high-purity bulk hexagonal diamond (HD) remains a formidable challenge, and a more detailed understanding of the transformation pathway from hexagonal graphite (HG) to HD remains elusive. Here, we elucidate the pathway selection mechanism and identify the favorable conditions for synthesizing high-purity bulk HD. Using deep potential molecular dynamics simulations, we systematically investigate the transformation under interlayer uniaxial compression across a broad temperature and pressure range. Our results demonstrate that interlayer stress governs the transformation pathway by regulating interlayer sliding of graphite layers. At low stress, the HG-to-HD transition proceeds through an intermediate orthorhombic graphite (OG) phase, whereas at high stress, it proceeds through a post-graphite phase; in both pathways, the final conversion to HD is completed via a buckling mechanism. Although temperature does not change the pathway type, it synergistically interacts with stress to regulate the formation of the AB′A-stacked OG phase, thereby determining the phase purity of HD. At 900 K and 55 GPa, an HD fraction of up to 86.7% is achieved. The interlayer stress-temperature map of the HD fraction exhibits an isolated island-like distribution, revealing an exceptionally narrow synthesis window. Moreover, the distribution of OG atom counts on the interlayer stress-temperature map provides a clear signature of the boundary separating the two transformation pathways. This work uncovers the pathway-selection mechanism for carbon phase transitions under extreme anisotropic stress conditions and establishes a theoretical framework for the controlled synthesis of high-purity bulk HD.