Liquid Carbon Structure Revealed for the First Time

In a groundbreaking scientific breakthrough, an international research team has for the first time experimentally measured the elusive state of liquid carbon, achieving an unprecedented glimpse into its atomic structure. This remarkable advance was accomplished through the innovative pairing of the cutting-edge high-power laser DIPOLE100-X with the ultrashort, intense X-ray laser pulses generated by the European XFEL facility located in Schenefeld, near Hamburg. The results, published recently in the prestigious journal Nature, open new avenues for understanding matter under extreme conditions and carry profound implications for planetary science and future energy technologies.

Carbon, one of the most fundamental elements to life and technology, has long mystified scientists when it comes to its behavior in liquid form. Unlike most materials, carbon does not simply melt under pressure; instead, under normal conditions, it sublimates directly from solid to gas. To transform carbon into a liquid state, extraordinarily high pressures and temperatures are necessary — approximately 4500 degrees Celsius, a temperature that exceeds the melting point of all known materials. This extreme environment has rendered laboratory studies of liquid carbon all but impossible until now, as no container material can withstand such conditions.

The research team overcame this monumental challenge by employing laser-driven compression techniques that induce phase changes on ultrafast timescales. Using the powerful DIPOLE100-X laser to generate shock waves, solid carbon samples were compressed and heated, briefly entering the liquid phase for mere billionths of a second. During this fleeting interval, European XFEL’s ultrashort X-ray pulses probed the atomic arrangement of the liquid carbon, an observation feat previously thought unachievable. The combination of laser-induced compression and X-ray diffraction thus provided a direct, time-resolved window into the liquid state.

This experimental setup represented a unique synergy between two state-of-the-art technologies. The DIPOLE100-X laser, developed by the UK’s Science and Technology Facilities Council, delivers high-energy pulses that precisely drive compression waves in the sample. Simultaneously, the European XFEL’s X-ray laser produces pulses lasting just quadrillionths of a second, allowing investigators to capture diffraction patterns before the sample relaxes or vaporizes. Such time-resolved diffraction data reveal how carbon atoms rearrange themselves as the material transitions from solid diamond-like order into a complex liquid structure.

Importantly, this experiment was conducted at the HED-HIBEF (High Energy Density Helmholtz International Beamline for Extreme Fields) station of the European XFEL, which was specifically designed for research involving extreme states of matter. The collaboration brought together numerous international institutions, combining expertise in laser physics, high-pressure science, and advanced X-ray diagnostics to tackle one of the longstanding frontiers of materials science.

Analysis of the diffraction patterns yielded surprising insights into the fundamental nature of liquid carbon. Contrary to earlier assumptions, the atomic structure of the liquid phase closely resembles that of solid diamond, exhibiting a coordination number of four — each carbon atom maintaining four nearest neighbors. This structural motif is reminiscent of water’s hydrogen bonding network, imparting liquid carbon with unique properties and complexity. The study thus confirms theoretical models and simulations that predicted such a water-like local order but lacked experimental validation until now.

Another critical achievement of the study was the precise determination of carbon’s melting point under extreme pressure. Prior theoretical approaches provided widely varying predictions, but the experimental data now substantially narrow this uncertainty. Accurately knowing the melting curve of carbon is essential not only for fundamental condensed matter physics but also for modeling planetary interiors and processes such as nuclear fusion, where carbon’s behavior under extreme conditions is pivotal.

The fleeting timescales of the experiments also highlight a new paradigm in high-pressure and high-temperature research. The entire laser-X-ray probing sequence lasts only nanoseconds, capturing snapshots of phase transitions as they happen. By systematically varying the delay between the compression pulses and X-ray shots, researchers generated a sequence of diffraction images that effectively stitch together the atomic rearrangements in real time. Through this approach, they constructed a dynamic “movie” of carbon’s transition from solid to liquid, a feat impossible through traditional static experiments.

Professor Dominik Kraus, who leads the Carbon Working Group within the collaboration, emphasized the exceptional nature of the findings: “For the first time, we can see direct experimental evidence of liquid carbon’s structure. It is a complex liquid with properties comparable to water, challenging our understanding of phase transitions at extreme states.” This breakthrough not only validates longstanding theoretical frameworks but also sets the stage for future explorations into exotic forms of matter.

Dr. Ulf Zastrau, head of the High Energy Density group at the European XFEL, underscored the significance of the research tools employed: “The combination of ultrafast, high-energy lasers with X-ray diffraction capabilities gives us a versatile toolkit to dissect matter under previously inaccessible conditions in extraordinary detail.” The capability to rapidly characterize material states under extreme pressure and temperature is poised to revolutionize multiple fields, from planetary science and astrophysics to advanced materials engineering.

Looking forward, the researchers anticipate that improvements in automation and data processing will dramatically accelerate such experiments. Currently, the acquisition and interpretation of data can take several hours, but enhanced computational frameworks may reduce this to a matter of seconds, enabling real-time experimentation and decision-making. This development will expand opportunities for probing a wider range of materials and phenomena, democratizing access to extreme matter research.

The success of this initial DIPOLE-XFEL experiment represents a landmark moment for science and technology. It underscores how state-of-the-art instrumentation and international collaboration can conquer experimental frontiers once deemed impossible. With liquid carbon finally accessible to direct study, new insights are expected to cascade into planetary geology, energy research, and condensed matter physics, fundamentally enriching our understanding of matter’s behavior at nature’s most extreme edges.

News Publication Date:
21-May-2025

Web References:
http://dx.doi.org/10.1038/s41586-025-09035-6

References:
D. Kraus, et al.: The structure of liquid carbon elucidated by in situ X-ray diffraction, Nature, 2025

Keywords

Liquid carbon, high-pressure physics, X-ray diffraction, European XFEL, laser compression, DIPOLE100-X, phase transition, diamond structure, ultrafast measurement, extreme matter, melting point, planetary interiors