Diamond has long been hailed as nature’s hardest substance, but that status may be about to change. A Chinese research team recently published a groundbreaking study in the journal Nature, successfully synthesizing a carbon material—hexagonal diamond (HD)—that is theoretically 60% harder than diamond. This achievement not only challenges conventional wisdom but also potentially opens up new avenues for the development of superhard materials and high-tech applications.
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The Difference Between Diamond and Hexagonal Diamond: Cubic vs. Hexagonal Structure
Traditional diamonds have a cubic lattice structure, with each carbon atom forming stable sp³ covalent bonds with four adjacent carbon atoms, creating a strong tetrahedral network. This arrangement gives diamonds their exceptional hardness and excellent physical properties.
However, as early as 60 years ago, scientists predicted another form of carbon structure: hexagonal diamond, which has a hexagonal lattice arrangement, unlike cubic diamond. In theory, this structure has superior mechanical properties, but it has always been difficult to synthesize in a pure state.
Natural HD: Evidence from Space
In nature, scientists have discovered trace amounts of hexagonal diamond at meteorite impact sites. For example, Lonsdaleite, a naturally occurring form of HD, is found in Diablo Crater in Arizona, USA. These minerals are often formed in the high-temperature and high-pressure environments created when meteorites impact graphite.
However, these natural samples are often mixed with other carbon phases, such as graphite and cubic diamond, and have extremely small particles and very low purity, making them unusable in practical fields of materials science.
Research breakthrough: Successful synthesis of pure bulk hexagonal diamond
After eight years of repeated experiments, a research team at the Beijing High Pressure Science Research Center has finally synthesized a block of HD crystals with a purity close to 100%. Measuring 1 mm in diameter and 70 microns thick, this is the largest and purest experimental sample to date.
Using high-quality single-crystal graphite as starting material, the researchers applied highly uniform pressure and high temperature in a specialized diamond anvil cell (DAC) and multi-anvil press to simulate the environment of a meteorite impact. They also used synchrotron radiation and electron microscopy to confirm the structure. This marks the first successful indoor production of large, fully structured HD crystals.
The secret of molecular structure: stronger interlayer bonds
Using advanced techniques such as Raman and infrared spectroscopy, the team discovered that all carbon bonds in the HD structure are sp³ σ bonds, with no sp² π bonds (characteristic of graphite). Particularly noteworthy is that one type of bond between the layers of the HD structure is significantly shorter than the other three types. This indicates stronger interlayer forces, which contributes to its high hardness.
In Vickers hardness tests, HD diamond exhibits significantly higher resistance to plastic deformation than conventional diamonds, proving that its superhard properties are not only theoretical but can also be verified experimentally.
Potential applications of HD: from drilling tools to quantum technology
If HD materials can be manufactured in even thicker and larger volumes in the future, their potential applications will be vast. First, they could be used as superhard tool materials in geothermal drilling, deep mineral extraction, and highly wear-resistant mechanical components.
In addition, due to its high mechanical stability and perfect sp³ structure, HD may also play a key role in cutting-edge technologies such as quantum computing devices, microelectromechanical systems (MEMS) and high-frequency electronic components.
It’s More Than Just a Diamond: A Milestone in Science and the Future of Materials
The name diamond comes from the Greek word “adámas,” meaning “unconquerable.” The discovery of hexagonal diamond may redefine our understanding of the “hardest substance.” It’s not only a milestone in materials science, but also symbolizes another breakthrough in humanity’s ability to challenge the limits of nature.
By further optimizing process conditions, improving precursor purity, and regulating parameters such as pressure and temperature, it is hoped that this carbon material, which is harder than diamond, can be mass-produced in the future, laying a more solid foundation for high-tech industry and the quantum era.
References:
- Nature (2025). DOI: 10.1038/s41586-025-09343-x
- 60% harder than diamond, scientists successfully synthesize bulk hexagonal diamond
- Scientists design superdiamonds with theoretically predicted hexagonal crystal structure
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