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Scientists measured carbon at pressures reaching 2,000 GPa

Carbon is one of the most ubiquitous elements. Besides being a building block for all known life, carbon material sits in carbon-rich exoplanets’ interior, hence subject to an intense investigation by scientists.

Various studies have shown that carbon’s crystal structure has a significant impact on material properties. In the elemental form, it is found in multiple allotropes, including graphite, diamond, and fullerenes, and it has long been predicted that even more structures can exist at pressures greater than those at Earth’s core above 1,000 gigapascals (GPa).

In a new study, scientists led by LLNL and the University of Oxford have successfully measured carbon at pressures reaching 2,000 GPa (five times the pressure in Earth’s core), nearly doubling the maximum pressure at which a crystal structure has ever been directly probed.

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

Professor Justin Wark from the University of Oxford said, “The NIF Discovery Science program is immensely beneficial to the academic community — it not only allows established faculty the chance to put forward proposals for experiments that would be impossible to do elsewhere but importantly also gives graduate students, who are the senior scientists of the future, the chance to work on a unique facility.”

For the study, scientists used the unique high power and energy and accurate laser pulse-shaping of LLNL’s National Ignition Facility to compress solid carbon to 2,000 GPa using ramp-shaped laser pulses. At the same time, they measured the crystal structure using an X-ray diffraction platform to capture a nanosecond-duration 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 result surprised scientists: when subjected to these intense conditions, solid carbon retains its diamond structure far beyond its regime of predicted stability. It confirms the predictions that the strength of the molecular bonds in diamond remains stable under enormous pressure, resulting in large energy barriers that hinder conversion to other carbon structures.

Jenei said, “Whether nature has found a way to surmount the high energy barrier to the formation of the predicted phases in the interiors of exoplanets is still an open question. Further measurements using an alternate compression pathway or starting from an allotrope of carbon with an atomic structure that requires less energy to rearrange will provide further insight.”

Co-authors include David Braun, Damian Swift, Martin Gorman, Ray Smith, Dayne Fratanduono, Federica Coppari, Christopher Wehrenberg, Rick Kraus, David Erskine, Joel Bernier, James McNaney, Robert Rudd, and Jon Eggert of LLNL; David McGonagle, Patrick Heighway, Matthew Suggit and Justin Wark of the University of Oxford; Ryan Rygg and Gilbert Collins of the University of Rochester’s Laboratory for Laser Energetics; and Andrew Higginbotham of the University of York.

Journal Reference:
  1. Lazicki, A., McGonagle, D., Rygg, J.R., et al. Metastability of diamond ramp-compressed to 2 terapascals. Nature 589, 532–535 (2021). DOI: 10.1038/s41586-020-03140-4

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