Carbon at pressure five times that of the earth’s core, breaks the record of crystal formation

Carbon, the fourth most abundant element in the universe, is a building block for all known life and a material that resides inside carbon-rich exoplanets.

Decades of intense research by scientists have shown that carbon’s crystal structure has a significant impact on its properties. In addition to graphite and diamond, the most common carbon structures found at ambient pressure, scientists have predicted several new carbon structures that can be found at pressures greater than 1000 gigapascals (GPa). This pressure, about 2.5 times the pressure in the Earth’s core, is relevant for modeling the inside of the exoplanet, but it is impossible to achieve in the laboratory.

That is, until now. In the Discovery Science program, which gives academic scientists access to LLNL’s flagship National Ignition Facility (NIF), an international team of researchers led by LLNL and the University of Oxford successfully measured carbon at pressures reaching 2000 GPa (5 times the pressure in the earth’s pressure (core), which almost doubles the maximum pressure at which a crystal structure has ever been directly examined. The results are in today Nature.

“We found that carbon, under these conditions, surprisingly does not change to any of the predicted phases, but that the diamond structure retains to the highest pressure,” said Amy Jenei, LLNL physicist, lead author of the study. “The same ultra-strong interatomic bonds (which require high energy to break) that are responsible for the metastable diamond structure of carbon that persists at ambient pressure indefinitely also likely to hinder its transformation above 1,000 GPa in our experiments.”

The academic component of the collaboration was led by Oxford professor Justin Wark, who praised the Lab policy for open access.

“The NIF Discovery Science program is very beneficial to the academic community,” he said. “It not only gives the established faculty the opportunity to submit proposals for experiments that would be impossible elsewhere, but more importantly, gives graduate students, who are the senior scientists of the future, the opportunity to come up with a completely unique facility. to work.”

The team, which also included scientists from the University of Rochester’s Laboratory of Laser Energetics (LLE) and the University of York, used the unique high power and energy of NIF and the accurate shape of laser pulse to produce solid carbon up to 2000 GPa. compressing using ramp-shaped laser pulse. This allowed them to measure the crystal structure using an X-ray diffraction platform and take a snapshot of the atomic grid of nanoseconds. These experiments almost double the record high pressure at which X-ray diffraction was recorded on any material.

The researchers found that solid carbon retains the diamond structure of solid carbon except under these intense conditions, even beyond the predicted stability, which confirms predictions that the strength of the molecular bonds in diamond persists under enormous pressure. This leads to large energy barriers that hinder the conversion to other carbon structures.

“Whether nature has found a way to overcome the high energy barrier for the formation of the predicted phases in the interior of exoplanets is still an open question,” Jenei said. “Further measurements using an alternative compression path or from an allotropic carbon with an atomic structure that requires less energy to rearrange will provide further insight.”