Structure of liquid carbon measured for the first time

An international research collaboration headed by the University of Rostock and the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) managed to study liquid carbon. This groundbreaking experiment was made possible at the European XFEL, the world's largest X-ray laser.

Liquid carbon can be found, for example, in the interior of planets and plays an important role in future technologies like nuclear fusion. To date, however, only very little was known about carbon in its liquid form because in this state it was practically impossible to study in the lab: Under normal pressure carbon does not melt but 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. No container would withstand that.

Laser compression, on the other hand, can turn solid carbon into liquid for fractions of a second. And the challenge was to use these fractions of a second to take measurements. In a previously unimaginable way, this has now become reality at the European XFEL, the world’s largest x-ray laser with its ultrashort pulses, in Schenefeld, near Hamburg. 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 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.

Structure of liquid carbon measured for the first time (HZDR)

Publication:
D. Kraus, et al.: The structure of liquid carbon elucidated by in situ X-ray diffraction, in Nature, 2025 (DOI: 10.1038/s41586-025-09035-6)

21.5.2025