A First Look at Liquid Carbon

Scientists working at one of the world's leading X-ray lasers have used very high pressures and temperatures to elucidate for the first time the liquid structure of one of the most common elements in the universe—carbon (Nature, doi: 10.1038/s41586-025-09035-6). They say that their result could improve the synthesis of novel materials and raise the efficiency of nuclear fusion reactions.

Simulations to experiments

The various forms of solid carbon, such as diamond and graphite, are familiar and used widely in technology. But the structure and properties of carbon’s liquid form, which requires temperatures of at least 4000 K and pressures of several hundred atmospheres or more, remain largely unknown. Liquid carbon is thought to exist in the center of large planets, but it is also created briefly during the synthesis of carbon nanotubes, nanodiamonds and other advanced materials. Knowledge of its behavior when compressed might also lead to more efficient implosions in laser-based fusion, given diamond’s use as an ablator around fuel pellets.

Researchers have assumed that transient chemical bonds existing in solid carbon lattices will persist in the liquid state, leading to the same basic molecular unit—one carbon atom linked to its four nearest neighbors in the shape of a tetrahedron. But because those units link up in a complex way, modeling the resulting structure is tricky. Simulating from first principles using density functional theory has given increasingly consistent results for carbon’s extended phase diagram, but experimental confirmation has been lacking.

Producing liquid carbon in static experiments by crushing samples between the tips of diamond anvil cells is very hard to do because the necessary high temperatures tend to destroy the cells and there is confusion between the carbon sample and carbon apparatus. Scientists therefore tend to perform dynamic experiments involving flash heating or shock compression. But incorporating an X-ray probe to study the fleeting liquid state is far from easy, meaning the evidence for that state tends to be indirect.

The peaks were most prominent at around 100 GPa (equivalent to a million atmospheres), then progressively diminished while the smoother features of liquid carbon emerged.

Creating the elusive liquid

In the latest work, an international collaboration led by Dominik Kraus at the University of Rostock and the Helmholtz-Zentrum Dresden-Rossendorf, both in Germany, was able to overcome these problems at the European X-Ray Free-Electron Laser (XFEL) facility. Kraus and colleagues used the DiPOLE100-X laser to produce pulses around 10 ns long, each with some 30 J of energy, to drive shock waves and create simultaneous heating in samples of glassy carbon. During the few nanoseconds that the carbon remained liquid, they exposed it to 18-keV XFEL pulses lasting just a few femtoseconds each, and then they captured the scattered light to deduce the liquid structure.

The researchers carried out their experiment at different pressures by varying the energy of the drive laser. At each pressure, they fired multiple shots to yield an averaged X-ray diffraction pattern at that pressure. The diffraction pattern came in the form of a graph with (or without) peaks that they could use to deduce the form of carbon present under those conditions.

At standard atmospheric pressure, the scientists recorded three broad peaks associated with the amorphous structure of glassy carbon. As they ramped up the pressure, they saw these peaks disappear and be replaced with the sharper peaks characteristic of crystalline diamond. The peaks were most prominent at around 100 GPa (equivalent to a million atmospheres), then progressively diminished while the smoother features of liquid carbon emerged. At 160 GPa, the team observed a purely liquid state.

Predictions confirmed

By comparing their results with those obtained from density functional theory, Kraus and colleagues were able to establish how many nearest neighbors each carbon atom had in the purely liquid state. They confirmed about four to be the correct value and found that a simpler model requiring higher numbers of nearest neighbors was at odds with their experimental measurements.

The researchers also used features of their diffraction patterns to establish the temperature and density of both the pure liquid state and a state containing liquid and solid. The resulting temperatures—between 6000 K and 7000 K in each case—agreed well with predictions of melting points from density functional theory.

Kraus and colleagues add that they set up each experimental shot manually, which meant a delay of around a minute from one shot to the next. In contrast, both the drive laser and X-ray probe are capable of firing about 10 times a second. Harnessing that repetition rate through automation, they say, would lead to more precise measurements and could reveal “the liquid structure of a plethora of compounds made out of light elements at extreme pressure and temperature conditions.”