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In materials science, the hardness of a material measures its resistance to deformation by indentation, scratching or other physical actions. By this metric, plastics and soft tissues are said to be very soft, whereas metals and hard minerals such as quartz are significantly harder.

Diamond is widely considered to be the hardest material in existence – only being able to be scratched by other diamonds – followed by the synthetic material boron nitride. But now, an international research team led by researchers from the University of Edinburgh, Scotland and experts from the University of Bayreuth, Germany and the University of Linköping, Sweden, reports a new breakthrough – a material that could rival diamond.

Their synthesis of ultra-incompressible and superhard carbon nitride materials has been published in the journal Advanced Materials.

The search for ultrahard materials

In 1989, leading materials scientists published a paper in Science predicting that “hypothetical covalent solids formed between carbon and nitrogen” would be “good candidates for extreme hardness.”

Carbon nitride (C3N4) compounds featuring a three-dimensional network of corner-sharing CN4 tetrahedra would then become one of the great aspirations in materials science, with hypothetical predictions expecting the compounds to have a hardness comparable to – or greater than – diamond.

In parallel, a growing demand for multifunctional materials in industry would drive scientists to more closely investigate these theoretical superhard compounds. Such materials could be used in protective coatings for cars and spaceships, high-endurance cutting tools, solar panels and photodetectors, experts believe.

However, after more than three decades of research and multiple synthesis attempts, nobody was able to bring about unambiguous evidence proving the synthesis of these hard carbon nitride compounds.

Now, researchers report the successful creation of three carbon nitride samples that exhibit extreme hardness and ultra-incompressibility.

Synthesized carbon nitride compounds rival diamond and boron nitride

To create the groundbreaking new materials, the researchers loaded various forms of carbon nitrogen precursors into laser-heated diamond anvil cells, subjecting them to extreme pressures of up to 1 million atmospheres and temperatures near 2500 Kelvin.

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X-ray beam analysis on the compounds was carried out at three different particle accelerators – the European Synchrotron Research Facility in France, the Deutsches Elektronen-Synchrotron in Germany and the Advanced Photon Source based in the United States – to allow the researchers to study the crystal structure of the compounds formed under these conditions.

They found that the carbon nitride materials produced did indeed have the necessary building blocks to superhardness.

“Upon the discovery of the first of these new carbon nitride materials, we were incredible to have produced materials researchers have been dreaming of for the last three decades,” said Dr. Dominque Laniel, UKRI Future Leaders Fellow in the University of Edinburgh’s Institute for Condensed Matter Physics and Complex Systems. “These materials provide [a] strong incentive to bridge the gap between high pressure materials synthesis and industrial applications.”

When the materials were returned to ambient temperature and pressure, the researchers conducted physical properties testing using optical and scanning electron microscopes (SEM). The team found dents in the diamond anvil cells, suggesting that the new carbon nitrides were hard enough to deform diamond.

Additional testing and computational calculations suggested that the new materials could also display other interesting properties, such as photoluminescence and high energy density, where a large amount of energy can be stored in a small amount of mass.

“These materials are not only outstanding in their multi-functionality, but show that technologically-relevant phases can be recovered from a synthesis pressure equivalent to the conditions found thousands of kilometers in the Earth’s interior,” said Dr. Florian Trybel, assistant professor in the Linköping University Department of Physics, Chemistry and Biology. “We strongly believe this collaborative research will open up new possibilities for the field.”

References: Laniel D, Trybel F, Aslandukov A, et al. Synthesis of ultra‐incompressible and recoverable carbon nitrides featuring CN4 tetrahedra. Adv Mater. 2023. doi: 10.1002/adma.202308030

This article is a rework of a press release issued by the University of Edinburgh. Material has been edited for length and content.

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