Researchers evaluate paths to create super diamonds

BC8 is believed to be found in the centre of carbon-rich exoplanets, whose presence is plausible based on recent astrophysical observations. These celestial bodies, characterized by considerable mass, experience gigantic pressures reaching millions of atmospheres in their deep interiors. 

“The extreme conditions prevailing within these carbon-rich exoplanets may give rise to structural forms of carbon such as diamond and BC8,” Ivan Oleynik, a physics professor at the University of South Florida and senior author of the article, said in a media statement. “Therefore, an in-depth understanding of the properties of the BC8 carbon phase becomes critical for the development of accurate interior models of these exoplanets.”

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

According to Oleynik and his colleagues, the most important reason that diamond is so hard is that the tetrahedral shape of the four-nearest-neighbour atoms in the diamond structure perfectly matches the optimal configuration of the four valence electrons in column-14 elements in the periodic table (beginning with carbon, followed by silicon and germanium).

“The BC8 structure maintains this perfect tetrahedral nearest-neighbour shape, but without the cleavage planes found in the diamond structure,” Jon Eggert, co-author of the paper, said. “The BC8 phase of carbon at ambient conditions would likely be much tougher than diamond.”

Through multi-million atomic molecular-dynamics simulations using the fastest exascale supercomputer in the world, the team uncovered the extreme metastability of the diamond at very high pressures, significantly exceeding its range of thermodynamic stability.

The key to this success was the development of very accurate machine-learning interatomic potential that describes interactions between individual atoms with unprecedented quantum accuracy at a wide range of high-pressure and temperature conditions.

“By efficiently implementing this potential on GPU-based (graphics processing unit) Frontier, we can now accurately simulate the time evolution of billions of carbon atoms under extreme conditions at experimental time and length scales,” Oleynik said. “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.”

In the researchers’ view, the significance of this study is twofold. First, it elucidates 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 synthesized within a very narrow range of pressures and temperatures.

Additionally, it predicts viable compression pathways to access this highly restricted domain where BC8 synthesis becomes achievable. Oleynik, Eggert, and others are collaborating to explore these theoretical pathways using Discovery Science shot allocations at the National Ignition Facility, a laser-based inertial confinement fusion research device located at the Lawrence Livermore National Laboratory in California.

The team dreams of, one day, growing a BC8 super diamond in the laboratory if only they could synthesize the phase and then recover a BC8 seed crystal back to ambient conditions.