Carbon at pressures 5 times those of the Earth’s core breaks crystal formation record

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

Decades of extensive research by scientists have shown that the crystal structure of carbon has a significant impact on its properties. In addition to graphite and diamond, the most common carbon structures found at ambient pressures, scientists have predicted several new carbon structures that could be found at pressures above 1,000 gigapascals (GPa). These pressures, about 2.5 times the pressure in the Earth’s core, are relevant for modeling the interiors of exoplanets but have been impossible to achieve in the laboratory.

That is, until now. As part of the Discovery Science program, which gives university scientists access to LLNL’s flagship National Ignition Facility (NIF), an international team of researchers led by the LLNL and the University of Oxford has successfully measured carbon at pressures up to 2000 GPa (5 times the pressure of Earth. core), almost doubling the maximum pressure at which a crystal structure has ever been directly probed. The results were reported today in Nature.

“We have found that, surprisingly, under these conditions, the carbon does not transform in any of the predicted phases but retains the structure of the diamond up to the highest pressure,” said Amy Jenei, LLNL physicist, author principal of the study. “The same ultra-strong interatomic bonds (requiring high energies to break) that are responsible for the diamond’s metastable structure of carbon which persists indefinitely at ambient pressure, probably also hinder its transformation above 1000 GPa in our experiments.

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

“The NIF Discovery Science program is of tremendous benefit to the academic community,” he said. “This not only allows established professors to present proposals for experiments that would be impossible to do elsewhere, but, importantly, also gives graduate students, who are the seasoned scientists of the future, the chance to work on a facility. totally unique. “

The team, which also included scientists from the Laboratory for Laser Energetics (LLE) at the University of Rochester and the University of York, took advantage of the exceptional power and energy of the NIF and the setting Precisely shapes laser pulses to compress solid carbon at 2000 GPa using a ramp-shaped laser. legumes. This allowed them to measure the crystal structure using an x-ray diffraction platform and capture a nanosecond snapshot of the atomic lattice. These experiments nearly double the record high pressure at which X-ray diffraction has been recorded on any material.

The researchers found that even under these intense conditions, solid carbon retains its diamond structure well beyond its predicted stability regime, confirming predictions that the strength of diamond molecular bonds persist under tremendous pressure. This results in large energy barriers that hinder conversion to other carbon structures.

“Whether nature has found a way to overcome the high-energy barrier to the formation of predicted phases inside exoplanets is still an open question,” Jenei said. “Further measurements using an alternative compression pathway or from an allotrope of carbon with an atomic structure that requires less energy to reorganize will provide additional information.”