Structure of liquid carbon
May 2025. HotCores members are part of Nature publication in which a team of an international research collaboration headed by the University of Rostock and the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) to publish the structure of liquid carbon
Groundbreaking experiment at European XFEL: Research team measured structure of liquid carbon for the first time (illustration: Martin Kuensting / HZDR)
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.
In this study, we are part of a community of leading international research institutions at the HED-HIBEF (High Energy Density) experimental station to combined powerful laser compression with high-performance DIPOLE100-X optical laser with ultrafast X-ray analysis and large-area X-ray detectors at the European XFEL, the world’s largest X-ray laser with its ultrashort pulses. We use the facility to send compression waves through a solid carbon sample and liquefy the material for a few nanoseconds and use the ultrashort X-ray laser flash of the European XFEL to collect a diffraction pattern and draw inferences about the current arrangement of the atoms in the liquid carbon. The experiment is then repeated with a slightly delayed X-ray pulse or under slightly different pressure and temperature conditions to trace the transition from solid to liquid phase one step at a time.
The measurements revealed that with four nearest neighbors each, the systemics of liquid carbon are similar to solid diamond, and precisely narrow down the melting point. Diamond formation is observed above about 76 GPa, the coexistence of diamond and liquid carbon from about 100 GPa and complete melting at about 160 GPa, with a first liquid observed at about 6,500 K.
For more on the topic :
- the press release of the European XFEL : Structure of liquid carbon measured for the first time ;
- the publication : Kraus, D., Rips, J., Schörner, M. et al. The structure of liquid carbon elucidated by in situ X-ray diffraction. Nature (2025). https://doi.org/10.1038/s41586-025-09035-6.