Carbon probed at record pressure

27 January 2021

An artist’s rendering of 55 Cancri e, a carbon-rich exoplanet. Credit: ESA/Hubble/M. Kornmesser

In a historic first, a team of scientists have successfully measured the structure of carbon at pressures reaching 2,000 gigapascals (GPa) – five times the pressure in Earth’s core and nearly doubling the maximum pressure at which a crystal structure has ever been directly probed.

The group, led by Lawrence Livermore National Laboratory (LLNL) and the University of Oxford, used the LLNL flagship Nation Ignition Facility (NIF) in their work, which is the largest laser system in the world. The results are published in Nature.

Carbon is one of the most ubiquitous elements in existence. As the fourth most abundant element in the universe, a building block for all known life and a material that sits in the interior of carbon-rich exoplanets, the element has been subject to intense investigation by scientists.

Achieving the impossible

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

The group were able to use the NIF thanks to LLNL’s Discovery Science programme. Professor Justin Wark from Oxford’s Department of Physics comments: ‘The NIF Discovery Science programme is immensely beneficial to the academic community, providing access to the largest laser system on earth, to create conditions that only exist elsewhere in the Universe. Creating solid matter at such pressures can only be achieved with laser compression techniques, and a new field of research is emerging into the study of matter at high pressures and densities, conditions which are prevalent in many planets beyond our own.’

A unique facility

The team leveraged the unique high power and energy and accurate laser pulse-shaping of the facility to compress solid carbon to 2,000 GPa using ramp-shaped laser pulses. They simultaneously measured the crystal structure using an X-ray diffraction platform to capture a nanosecond-duration snapshot of the atomic lattice.

The researchers found that even when subjected to these intense conditions, solid carbon retains its diamond structure far beyond its regime of predicted stability, confirming predictions that the strength of the molecular bonds in diamond persists under enormous pressure, resulting in large energy barriers that hinder conversion to other carbon structures.

The group also included scientists from the University of Rochester’s Laboratory for Laser Energetics and the University of York.

Metastability of diamond ramp-compressed to 2 terapascals, A Lazicki et al, Nature 589, 27 January 2021

Read LLNL article.

Image: An artist’s rendering of 55 Cancri e, a carbon-rich exoplanet. Credit: ESA/Hubble/M. Kornmesser