Supercomputer breaks down how to make material harder than diamond: the ‘super diamond’

Diamonds are known as the hardest substances in existence, a status that has recently encountered some challenge. In theory, a variation in the way carbon atoms are arranged would make it even harder, but so far no one has achieved the pressure needed to create a so-called “super diamond.” That could be about to change, however, as computer models indicate the conditions that may be needed.

The power of diamonds comes from the way each carbon atom is connected to its four nearest neighbors by covalent bonds, creating exceptionally tightly packed atoms. It is known that pure carbon atoms can arrange themselves in many ways, which is why you get materials like buckyballs and graphene.

There is still debate about whether exotic materials can outperform diamonds using boron nitride there is a way to connect carbon atoms that is theoretically stronger than traditional diamond arrangements. This is known as body-centered eight-atom cube (BC8) and is estimated to be 30 percent more resistant to compression than regular diamonds.

There would certainly be an industrial demand for such a material, but it is thought that a pressure of at least 10 million atmospheres (one trillion Pascals) would be required to form the atoms into such a shape. However, once created they should be stable under more normal conditions. Labs have managed to achieve the conditions that some previous estimates predicted would produce BC8 diamonds, but found these were too optimistic, leaving scientists wondering how high the pressure should be.

If humans haven’t been able to reach such extreme conditions to create super diamonds, you might expect nature wouldn’t either. That’s probably true on Earth, but it’s thought that some exoplanets (planets orbiting other stars) could be very carbon-rich. The pressure at the center of worlds like this could easily be enough to meet the demands.

“The extreme conditions within these carbon-rich exoplanets can give rise to structural forms of carbon such as diamond and BC8,” said professor Ivan Oleynik of the University of South Florida in a statement. “Therefore, a deep understanding of the properties of the BC8 carbon phase becomes critical for the development of accurate interior models of these exoplanets.”

BC8 can also occur in silicon and germanium, the elements immediately below carbon in the periodic table, and these have been produced. Using what we know about its production in these elements, Oleynik and colleagues created computer models to investigate what it would take for this to happen in carbon.

The calculations involved are enormous, but with the help of Frontier, the world’s fastest exascale supercomputer, the team believes they have identified what it takes to make billions of atoms come together in the desired way. “We predicted that the post-diamond BC8 phase would be experimentally accessible only within a narrow high-pressure, high-temperature region of the carbon phase diagram,” Oleynik said.

Specifically, pressures of 1,050,000,000,000 Pascals would be needed at a precise temperature, probably around 6,000 K. Even higher pressures would increase the potential temperature range, but not by much. To achieve that, the program predicts that ordinary diamonds would melt into a metastable, supercooled carbon liquid, creating BC8s. Like ice particles in supercooled water, BC8 crystals would have great difficulty getting started, but once one formed, they would grow rapidly through nucleation.

Whether there is equipment on Earth that is up to the challenge of making this possible remains to be seen.

If that’s possible, the authors think BC8 carbon could do more than simply surpass diamonds’ resistance to pressure. “The BC8 structure retains this perfect nearest-neighbor tetrahedral shape, but without the cleavage planes found in the diamond structure,” said co-author Dr. Jon Eggert of Lawrence Livermore National Laboratory. Despite the enormous costs of creating something like this, that robustness could be invaluable, as well as provide lessons about the inner workings of planets with such cores.

The study was published in The Journal of Physical Chemistry Letters.

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