Researchers have been able to measure liquid carbon experimentally for the first time. They combined a high-power laser with the ultrashort X-ray laser flash of the European XFEL. Credit: HZDR / M. Künsting
Researchers have completed a groundbreaking experiment at the European XFEL.
An international team of scientists, led by the University of Rostock and the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), set out to investigate matter under extreme pressure. In 2023, they used the high-power DIPOLE 100-X laser at the European XFEL for the very first time, achieving remarkable results. Their groundbreaking experiment successfully captured the behavior of liquid carbon, a feat never before accomplished, as reported in the journal Nature.
Liquid carbon occurs naturally inside planets and could play a crucial role in future energy technologies such as nuclear fusion. Yet until now, researchers knew very little about this elusive state. The challenge lies in the fact that carbon does not melt under normal conditions. Instead, it bypasses the liquid phase entirely and turns directly into gas.
To transform into a liquid, carbon requires extreme pressures combined with temperatures of about 4,500 degrees Celsius, the highest melting point of any known material. Containing such conditions in a laboratory setting has long been impossible because no physical container could survive them.
Laser compression provided the breakthrough. By delivering an intense, short burst of energy, the DIPOLE 100-X laser was able to convert solid carbon into liquid for only fractions of a second. The true challenge was capturing data during that fleeting moment. Thanks to the unique capabilities of the European XFEL, the world’s most powerful X-ray laser located in Schenefeld near Hamburg, researchers were finally able to make those measurements. Its ultrashort X-ray pulses made it possible to probe the liquid state in real time, turning a once unimaginable experiment into reality.
Unique measuring technology in this combination
The unique combination of the European XFEL with the high-performance laser DIPOLE100-X was crucial for the success of the experiment. It was developed by the British Science and Technology Facilities Council and made available to scientists from all over the world by the HIBEF User Consortium (Helmholtz International Beamline for Extreme Fields). A community of leading international research institutions at the HED-HIBEF (High Energy Density) experimental station at the European XFEL has now combined powerful laser compression with ultrafast X-ray analysis and large-area X-ray detectors for the first time.
In the experiment, the high-energy pulses of the DIPOLE100-X laser drive compression waves through a solid carbon sample and liquefy the material for nanoseconds, that is, for a billionth of a second. During this nanosecond, the sample is irradiated with the ultrashort X-ray laser flash of the European XFEL. The carbon atoms scatter the X-ray light – similar to the way light is diffracted by a grating. The diffraction pattern allows inferences to be drawn about the current arrangement of the atoms in the liquid carbon.
The whole experiment only lasts a few seconds but is repeated many times: every time with a slightly delayed x-ray pulse or under slightly different pressure and temperature conditions. Many snapshots combine to make a movie. Researchers have thus been able to trace the transition from solid to liquid phase one step at a time.
Water-like structure and accurate melting point determined
The measurements revealed that with four nearest neighbors each, the systemics of liquid carbon are similar to solid diamond. “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,” explains the head of the research collaboration’s Carbon Working Group, Prof. Dominik Kraus from the University of Rostock and HZDR.
The researchers also managed to precisely narrow down the melting point. Up to now, the theoretical predictions on the structure and melting point had diverged significantly. But precise knowledge is crucial for planet modelling and certain concepts for power generation through nuclear fusion.
The first DIPOLE experiment at the European XFEL also ushers in a new era in measuring matter under high pressure, as HED group leader, Dr. Ulf Zastrau, emphasizes, “We now have the toolbox to characterize matter under highly exotic conditions in incredible detail.” And the experiment’s potential is far from being exhausted. In the future, results that currently take several hours’ experiment time could be available in a few seconds – as soon as the complex automatic control and data processing work fast enough.
Reference: “The structure of liquid carbon elucidated by in situ X-ray diffraction” by D. Kraus, J. Rips, M. Schörner, M. G. Stevenson, J. Vorberger, D. Ranjan, J. Lütgert, B. Heuser, J. H. Eggert, H.-P. Liermann, I. I. Oleynik, S. Pandolfi, R. Redmer, A. Sollier, C. Strohm, T. J. Volz, B. Albertazzi, S. J. Ali, L. Antonelli, C. Bähtz, O. B. Ball, S. Banerjee, A. B. Belonoshko, C. A. Bolme, V. Bouffetier, R. Briggs, K. Buakor, T. Butcher, V. Cerantola, J. Chantel, A. L. Coleman, J. Collier, G. W. Collins, A. J. Comley, T. E. Cowan, G. Cristoforetti, H. Cynn, A. Descamps, A. Di Cicco, S. Di Dio Cafiso, F. Dorchies, M. J. Duff, A. Dwivedi, C. Edwards, D. Errandonea, S. Galitskiy, E. Galtier, H. Ginestet, L. Gizzi, A. Gleason, S. Göde, J. M. Gonzalez, M. G. Gorman, M. Harmand, N. J. Hartley, P. G. Heighway, C. Hernandez-Gomez, A. Higginbotham, H. Höppner, R. J. Husband, T. M. Hutchinson, H. Hwang, D. A. Keen, J. Kim, P. Koester, Z. Konôpková, A. Krygier, L. Labate, A. Laso Garcia, A. E. Lazicki, Y. Lee, P. Mason, M. Masruri, B. Massani, E. E. McBride, J. D. McHardy, D. McGonegle, C. McGuire, R. S. McWilliams, S. Merkel, G. Morard, B. Nagler, M. Nakatsutsumi, K. Nguyen-Cong, A.-M. Norton, N. Ozaki, C. Otzen, D. J. Peake, A. Pelka, K. A. Pereira, J. P. Phillips, C. Prescher, T. R. Preston, L. Randolph, A. Ravasio, D. Santamaria-Perez, D. J. Savage, M. Schölmerich, J.-P. Schwinkendorf, S. Singh, J. Smith, R. F. Smith, J. Spear, C. Spindloe, T.-A. Suer, M. Tang, M. Toncian, T. Toncian, S. J. Tracy, A. Trapananti, C. E. Vennari, T. Vinci, M. Tyldesley, S. C. Vogel, J. P. S. Walsh, J. S. Wark, J. T. Willman, L. Wollenweber, U. Zastrau, E. Brambrink, K. Appel and M. I. McMahon, 21 May 2025, Nature.