Carbon, one of the most abundant elements in the universe, constitutes many key components of life and technology. Because of this, the material is very well-studied - at least in its solid form. As a liquid, carbon structure is very difficult to measure because the state of matter only exists at extreme pressures and temperatures.
In a recent study, published in Nature, an international team including Lawrence Livermore National Laboratory (LLNL) researchers experimentally measured the structure of liquid carbon for the first time. The results, which agree with theoretical predictions, show carbon arranged in a tetrahedral bond structure, with each atom having four neighbors.
"We've proved that you can open up a whole new area of research - low atomic number liquids at high pressures - that everyone wants to go to, but haven't been able to so far," said LLNL scientist and author Christopher McGuire.
The experiments were conducted with the DiPOLE 100-X high-energy laser at the European X-ray Free-Electron Laser (XFEL) facility. DiPOLE was used to shock compress and liquify a glassy carbon sample, which was specifically chosen to melt at an accessible pressure.
While DiPOLE created the liquid carbon, the XFEL system measured its structure. XFELs accelerate electrons to velocities near the speed of light before directing them through a long series of magnets that generate an oscillating field along the electron path. As they undulate within the magnetic field, the electrons emit trillions of photons in very short, intense and coherent pulses of X-ray light. Those X-rays were directed toward the liquid carbon, where they punched through the material, interacted with electrons and scattered.
The critical advance in this work was enabled by the extraordinary flux and energy of the XFEL and the large area detectors that captured the scattered X-rays. Combined, they provided the necessary signal level and coverage to make these first-in-class measurements possible.
To date, most work using X-ray diffraction to probe high-pressure materials has focused on solids and elements with relatively high atomic numbers. This is because in a solid, the planes of linked atoms act like small mirrors for the X-rays, creating a signal of distinct scattering angles and patterns that increases with atomic number.
"But liquids are much more difficult," said LLNL scientist and author Jon Eggert. "They don't have an ordered lattice, and they don't have ordered planes to act as mirrors, but the scattered X-rays still interfere with each other."
The liquid carbon showed the same signal in every direction: carbon atoms with four bonded neighbors. Most liquids are bonded to more than four other atoms, and the low number creates challenges for modeling.
"If you take a ball, the closest number of touching balls that can surround it is 12. That's easy - we know exactly how the packing works," said LLNL scientist and author Saransh Soderlind.
With only four spheres, it's more complicated. There are more degrees of freedom and more chaos.
Chemistry calculations from electron orbitals predicted the tetrahedral arrangement, but this experimental validation lays a crucial foundation.
"Measurements of how a material responds to extreme conditions allow people that do calculations to have better constraints," said LLNL scientist and author Andy Krygier. "Just a little bit of data tells them a set of calculations is right, and now they can use that to take it to the next step and try to understand material properties that we will never measure."
Such models and measurements can be applicable for modeling the interiors of planets and understanding materials relevant to LLNL's stockpile stewardship mission.
The group aims to continue using next-generation XFEL sources for better measurements of temperature during dynamic compression experiments and for direct imaging.
"There's a lot of other experiments that XFELs can do, just like lasers can do a lot more than 'light shows'," said Eggert.
Other LLNL authors include Travis Volz, Suzanne Ali, Richard Briggs, Amy Coleman, Hyunchae Cynn, Martin Gorman, Trevor Hutchinson, Amy Lazicki, Kien Nguyen-Cong, Raymond Smith and Cara Vennari.
