Findings of ZZU team are published online in the journal Nature. [Photo/zzu.edu.cn]
A research team from Zhengzhou University (ZZU) has successfully synthesized bulk pure-phase hexagonal diamond and precisely resolved its crystal structure, revealing a novel phase transition mechanism. The findings were published online in the journal Nature on March 5, 2026, under the title "Bulk hexagonal diamond".
Diamond, renowned for its exceptional hardness, thermal conductivity, and wide bandgap, typically adopts a cubic structure. However, the existence of a hexagonal polymorph was first predicted theoretically in 1962 and later discovered in meteorites in 1967. Yet natural samples exist only as nanoscale grains embedded in meteorites, making isolation and property measurement extremely challenging. Moreover, the high formation energy barrier of hexagonal diamond under laboratory conditions has long hindered its synthesis, fueling debate over whether it can exist as a stable bulk material.
After five years of dedicated research, the team developed a large-volume uniaxial high-pressure apparatus and proposed a graphite layer-confined sliding strategy. Using highly oriented pyrolytic graphite as a precursor, they successfully produced a millimeter-sized pure hexagonal diamond at 20 GPa and 1300 C. Through synchrotron X-ray diffraction, aberration-corrected transmission electron microscopy, and electron energy loss spectroscopy, they obtained clear atomic-resolution images confirming its hexagonal structure.
Collaborating with Professor Sun Jian's team at Nanjing University, machine learning molecular dynamics simulations revealed an unprecedented phase transition pathway. Mechanical property testing showed that the synthesized hexagonal diamond exhibits Vickers hardness and shear modulus surpassing those of conventional cubic diamond.
This breakthrough not only achieves a decades-long scientific goal but also opens pathways for scalable production and future applications of hexagonal diamond.
Crystal structure, X-ray diffraction and atomic-level resolution images of synthesized hexagonal diamonds. [Photo/zzu.edu.cn]