Liquid carbon structure captured for first time using laser X-ray fusion

Liquid carbon exists deep within planetary interiors and may play a pivotal role in technologies such as nuclear fusion. However, examining it in the laboratory had proven elusive. At standard pressure, carbon skips the liquid state and vaporizes. Only at about 4,500 degrees Celsius and under immense pressure does carbon liquefy-conditions no conventional container can withstand.

The researchers circumvented this using laser compression, briefly transforming solid carbon into liquid. At the European XFEL in Schenefeld, ultrashort X-ray laser pulses allowed the team to take real-time measurements during these fleeting nanoseconds.

The experimental breakthrough hinged on coupling the XFEL's high-speed X-ray pulses with the DIPOLE 100-X laser developed by the UK's Science and Technology Facilities Council. This setup, hosted by the HIBEF User Consortium at the HED-HIBEF experimental station, enabled simultaneous laser compression and X-ray diffraction measurements for the first time.

During each run, the DIPOLE100-X laser sent shockwaves through carbon samples, liquefying them briefly. The XFEL then bombarded the liquid with X-ray flashes, and the resulting diffraction patterns revealed atomic arrangements in unprecedented detail. By varying pressure, temperature, and timing over many iterations, researchers effectively created a time-resolved movie of carbon transitioning from solid to liquid.

The study found liquid carbon exhibits a water-like structure with four nearest atomic neighbors-akin to diamond. "This is the first time we have ever been able to observe the structure of liquid carbon experimentally," said Prof. Dominik Kraus of the University of Rostock and HZDR. "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."

The team also refined the known melting point of carbon, addressing longstanding discrepancies between theory and simulation. This data is vital for planetary science and nuclear fusion modeling.

Dr. Ulf Zastrau, HED group leader, highlighted the broader implications: "We now have the toolbox to characterize matter under highly exotic conditions in incredible detail." The researchers anticipate faster results in the future as control systems and data processing improve.

Research Report:The structure of liquid carbon elucidated by in situ X-ray diffraction