An international research collaboration led by the University of Rostock and the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) used complementary technologies from STFC and the European X-ray Free Electron Laser (XFEL) to create and measure liquid carbon.
Liquid carbon is a substance that can be found in the interior of planets and plays an important role in future technologies like nuclear fusion.
The paper was published today, 21 May 2025, in the journal Nature (DOI: 10.1038/s41586-025-03035-6).
Complementary technologies
This experiment was only made possible through the combination of the UK-built DiPOLE 100X (D100X) laser and the European XFEL facility near Hamburg, Germany.
The D100X laser created conditions that liquifies solid carbon samples for billionths of a second, while the X-ray beam captured diffraction patterns that reveal the atomic arrangement in the liquid carbon.
Each experiment lasts only fractions of a second but is repeated many times with slightly different parameters. These snapshots of the diffraction patterns are then combined to create a comprehensive picture of carbon’s transition from solid to liquid phase.
Prof. John Collier, Director of STFC CLF, says:
"This discovery showcases UK capabilities at their finest. The DiPOLE 100-X system represents years of UK expertise enabling measurements once thought impossible. We’re proud that in partnering with our colleagues at the European XFEL, our technology is helping scientists from around the world understand these novel states of materials."
Extreme scientific challenge
Very little was known about carbon in its liquid form because it was practically impossible to study in the lab. Carbon does not melt under normal pressure, instead it immediately changes into a gaseous state.
Only under extreme pressure and at temperatures of approximately 4,500 degrees Celsius – the highest melting point of any material – does carbon become liquid.
On the other hand, laser compression can turn solid carbon into liquid for fractions of a second. The challenge was to use these fractions of a second to take measurements.
This has now become a reality at the European XFEL with the D100-X system, which was designed to study extreme states of matter, such as liquid carbon. A successful measurement
The measurements revealed that with four nearest neighbours each, the systemics of liquid carbon are like solid diamond. The researchers also precisely determined carbon’s melting point, resolving disagreements among previous theoretical predictions.
This precise knowledge, assisted by UK technology, is crucial for accurate planetary modelling and for developing certain concepts of power generation through nuclear fusion.
The STFC’s laser system has opened new research possibilities that were previously unimaginable.
In the future, results that currently require several hours of experiment time could be available in just seconds, once the complex automatic control and data processing systems are optimised.
Professor Dominik Kraus, head of the research collaboration’s Carbon Working Group from the University of Rostock and HZDR, explains:
"This is the first time we have ever been able to observe the structure of liquid carbon experimentally. Our experiment confirms the predictions made by sophisticated simulations of liquid carbon. We are looking at a complex form of liquid, comparable to water, that has very special structural properties."
DiPOLE-100 lasers soon available in the UK
The DiPOLE-100 laser technology will be available at the STFC’s new Extreme Photonics Application Centre (EPAC), Rutherford Appleton Laboratory, based at the Harwell Science and Innovation Campus.
This will allow UK researchers and international collaborators to conduct similar experiments right here in the UK. EPAC is due to come online in 2026.
