Supercomputer simulations of superdiamonds suggest a path to creating them

Diamond is the strongest material known. However, another form of carbon is expected to be harder than diamond. The challenge is how to create it on the ground.

The body-centered octahedral cubic crystal (BC8) is a distinct carbon phase: not diamond, but very similar. BC8 is expected to be a stronger material, exhibiting 30% greater compressive strength than diamond. It is believed to be found in the center of carbon-rich exoplanets. If BC8 can be recovered under ambient conditions, it can be classified as a super diamond.

The crystalline high-pressure phase of carbon is theoretically expected to be the most stable phase of carbon under pressures exceeding 10 million atmospheres.

“The BC8 phase of carbon at ambient conditions would be a new superhard material that will likely be stronger than diamond,” said Ivan Oleinik, a professor of physics at the University of South Florida (USF) and lead author of a recently published paper. in Journal of Physical Chemistry Letters.

“Despite numerous efforts to synthesize this elusive carbon crystalline phase, including previous National Ignition Facility (NIF) campaigns, it has never been observed,” said Marius Mellot, a Lawrence Livermore National Laboratory (LLNL) scientist who was also involved in the research. after”. “But we think it may be present on carbon-rich exoplanets.”

Recent astrophysical observations indicate the plausible existence of carbon-rich exoplanets. These celestial bodies, which are characterized by a large mass, are exposed to enormous pressures reaching millions of atmospheres in their deep interior.

“Consequently, the extreme conditions prevailing within these carbon-rich exoplanets may give rise to structural forms of carbon such as diamonds and BC8,” Oleinik said. “Therefore, an in-depth understanding of the properties of the BC8 carbon phase becomes crucial for developing accurate interior models of these exoplanets.”

BC8 is a high-pressure phase of both silicon and germanium that can be recovered at ambient conditions, and theory suggests that BC8 should also be stable at ambient conditions.

The most important reason why diamond is so hard is that the tetrahedral shape of the four neighboring atoms in diamond's structure matches exactly the optimal configuration of the four valence electrons in the elements of column 14 of the periodic table (starting with carbon, followed by silicon and germanium), said LLNL scientist and co-author John Eggert. .

“The BC8 structure maintains this perfect shape of the nearest tetrahedral neighbor, but without the cleavage planes found in the diamond structure,” Eggert said, agreeing with Oleinik that “the BC8 phase of carbon at ambient conditions would likely be much harder than diamond.”

Through atomic molecular dynamics simulations on Frontier, the world's fastest exascale supercomputer, the team discovered the extreme stability of diamond at very high pressures, well beyond the range of thermodynamic stability.

The key to success was the development of highly accurate interatomic capabilities for machine learning, which describe interactions between individual atoms with unprecedented quantitative precision over a wide range of high-pressure and temperature conditions.

“By effectively implementing this capability on the Frontier GPU, we can now accurately simulate the temporal evolution of billions of carbon atoms under extreme conditions at experimental time and length scales,” Olenik said. “We expected that the post-diamond BC8 phase would only be experimentally accessible within a narrow, high-pressure, high-temperature region of the carbon phase diagram.”

The importance is twofold. First, it explains the reasons behind the inability of previous experiments to synthesize and observe the elusive BC8 phase of carbon. This limitation arises from the fact that BC8 can only be manufactured within a very narrow range of pressures and temperatures.

In addition, the study predicts viable stress pathways to reach this highly restricted domain where BC8 synthesis becomes possible. Oleynik, Eggert, Millot, and others are currently collaborating to explore these theoretical pathways using Discovery Science snapshot allocations on NIF.

The team dreams of one day growing super-BC8 diamonds in the laboratory if they can only manufacture the stage and then recover the BC8 seed crystal and return it to ambient conditions.