Chinese Scientists Have Successfully Synthesized Hexagonal Diamond With A Size Of 100 Micrometers

After nearly ten years of continuous research, the international research team of Beijing High Voltage Science Research Center and Xi'an Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, has made a major breakthrough. Researchers have successfully transformed high-quality graphite single crystal precursor into hexagonal diamond at the micrometer level, and this achievement was published in the internationally authoritative journal Nature on the 30th.

 

Hexagonal diamond exhibits mechanical properties comparable to cubic diamond. Unlike cubic diamonds with only a single carbon carbon bond length, hexagonal diamonds exhibit two different bond length distribution characteristics. It is reported that the interlayer spacing is significantly shortened, and this unique stacking method of carbon atoms can effectively overcome the inherent weakness of sliding on the dense stacking surface of cubic diamond.

 

The synthesis of hexagonal diamond has always been a major challenge in the scientific community. As early as 1967, American scientists first discovered this rare "super diamond" in a meteorite crater, which attracted much attention due to its hexagonal crystal structure. However, the formation conditions of hexagonal diamonds are extremely strict, and previously they could only coexist with meteorites at the nanoscale.

 

The research team innovatively designed a high-temperature and high-pressure experimental plan. Scientists have conducted in-situ research on the structural changes of graphite under ultra-high pressure and high temperature conditions using laser heated diamond anvil technology. Experiments have found that graphite forms a "post graphite phase" high-pressure structure in the high-pressure range, and then hexagonal diamond is successfully obtained through local heating.

 

At the same time, the research team combined large-scale molecular dynamics theory simulations to reveal the crucial role of graphite layer stacking configuration in the formation of hexagonal diamond structures. This discovery confirms a new pathway for graphite to form hexagonal diamond through post graphite phase, opening up new technological directions for the preparation of superhard materials.

 

Experimental data shows that the synthesized hexagonal diamond has a hardness of up to 155GPa, exceeding natural diamond by more than 40%. In a vacuum environment, its thermal stability can reach 1100 ℃, which is significantly better than the 900 ℃ performance of nanodiamonds. These excellent physical properties make hexagonal diamond exhibit great potential in industrial applications.

 

This systematic study put an end to over 60 years of academic controversy regarding the macroscopic existence of hexagonal diamonds. The research results not only provide strong evidence for the independent existence of hexagonal diamond, but also lay a solid foundation for its development as a new generation of high-performance functional materials. This breakthrough is expected to drive technological innovation in the field of superhard materials and inject new impetus into the development of related industries.

 

What is  the Hexagonal diamond?

Hexagonal diamond (HD), also called lonsdaleite, is a hexagonal allotrope of carbon in which every carbon atom is sp³-hybridized and tetrahedrally bonded to four neighbors, forming a three-dimensional covalent framework. First predicted in 1962 and identified in 1967 in meteoritic material from Canyon Diablo (Arizona), it was named after crystallographer Kathleen Lonsdale. For decades only nanoscale fragments or disordered intergrowths were available, so its existence as a distinct phase and its intrinsic properties remained controversial. In 2025 a team led by Prof. Ho-kwang (Dave) Mao at the Center for High Pressure Science & Technology Advanced Research synthesized millimetre-sized, highly ordered single-phase lonsdaleite by compressing and heating high-quality graphite single crystals under controlled quasi-hydrostatic pressures. Electron microscopy revealed direct conversion of graphite (1010) into HD (0002) and graphite (0002) into HD (1010). The resulting samples are almost pure lonsdaleite with only trace cubic-diamond defects and exhibit a hardness slightly exceeding that of cubic diamond.

 

How is lonsdaleite different from cubic diamond?

Lonsdaleite (hexagonal diamond) differs from cubic diamond in four key ways:

1 Crystal symmetry and stacking
• Lonsdaleite: hexagonal lattice, space-group P6₃/mmc; the sp³-bonded carbon layers are stacked in an AB-AB sequence along the c-axis .
• Cubic diamond: face-centred-cubic lattice, space-group Fd-3m; layers follow an ABC-ABC sequence .

 

2 Lattice parameters
• Lonsdaleite: a ≈ 2.51 Å, c ≈ 4.12 Å .
• Cubic diamond: a = 3.5668 Å .

 

3 Stability and energy
• Lonsdaleite is ~0.025 eV per atom higher in energy than cubic diamond, making it metastable .

 

4 Mechanical properties
• Theory and recent mm-scale single-crystal syntheses indicate lonsdaleite is slightly harder (indentation resistance ~152 GPa, 58 %

harder on the {0001} plane vs diamond {111}) .
• Cubic diamond remains the benchmark for hardness (Mohs 10) and is more readily synthesized in bulk form.

 

Consequently, lonsdaleite's hexagonal stacking and slightly longer bond geometry give it marginally superior theoretical hardness, while cubic diamond offers greater thermodynamic stability and ease of production.