Scientists are able to study 'liquid carbon' in the lab for the first time ever

Liquid carbon has kept physicists guessing for decades. It forms only under crushing pressures and searing heat, and it never sticks around long in the lab.

In a new study, researchers finally captured the atomic structure of this short-lived liquid and mapped its melting conditions with precision.

They did it by squeezing and heating carbon for billionths of a second, then taking ultrafast x-ray snapshots before the state vanished.

Who led the work

The team was led by Prof. Dominik Kraus of the University of Rostock and the Helmholtz-Zentrum Dresden-Rossendorf (HZDR).

His group coordinated an international collaboration that ran the experiment at the European XFEL near Hamburg.

Kraus and colleagues report that the liquid holds a surprisingly ordered pattern. The atoms keep about four neighbors on average, which is similar to the arrangement in diamond.

They also anchored key numbers for the melting line at pressures close to a million atmospheres. That matters because many models of carbon at extreme conditions have disagreed for years.

Why liquid carbon matters

Carbon is central to electronics, energy technology, and the chemistry of life. Its solid forms range from diamond to graphite, but its liquid phase has been almost a blank page.

Planetary science needs that missing data. The team notes that liquid carbon’s properties help constrain interiors of sub-Neptune exoplanets and the magnetic behavior of Uranus and Neptune, where carbon may melt under deep pressure.

There is also a materials story. Liquid carbon is a transient step toward making nanodiamonds and other advanced carbon phases during rapid heating and cooling.

High energy facilities care too. Carbon shows up in lasers and fusion targets, so knowing its structure at extreme pressure feeds directly into more accurate simulations.

Making liquid carbon in the lab

The experiment paired a high-power optical laser with an x-ray free-electron laser. An x-ray free-electron laser produces ultrashort, bright x-ray pulses from electrons accelerated to near light speed, which are ideal for probing atoms in motion.

The optical drive was the DiPOLE 100-X system integrated with the High Energy Density instrument at European XFEL. Its pulse shaping and timing let researchers launch clean compression waves into a thin carbon target.

Each compression brought the sample to extreme pressure and high temperature for only a few nanoseconds. During that window, an x-ray pulse recorded a diffraction pattern that preserved the liquid’s structure.

The team repeated this sequence with slightly different timings, pressures, and temperatures. The result was a time-ordered set of snapshots spanning solid carbon, mixed solid-liquid states, and a fully liquid state.

What the x-rays measured

X-ray diffraction reveals how atoms are arranged. The scattered pattern is converted into a static structure factor, often written S(k), which encodes distances and correlations among atoms.

From S(k), scientists compute the radial distribution function, g(r), which shows how likely it is to find an atom a certain distance from a reference atom.

Integrating the first peak of g(r) gives the coordination number, the count of nearest neighbors in the liquid.

The data also respond to temperature and density. As temperature rises, peaks broaden, and as density increases, peaks shift to higher scattering vectors.

The group compared their measurements with DFT-MD simulations, which is density functional theory combined with molecular dynamics. That match anchored estimates of temperature and density and validated the microscopic picture.

What the data says

The structure is not like a simple liquid with a dozen neighbors around each atom. Instead, it clusters near four, echoing the tetrahedral bonding seen in solid diamond at ambient conditions.

“Our results show a complex fluid with transient bonding and approximately four nearest neighbours on average. Here we present a precise structure measurement of liquid carbon at pressures of around 1 million atmospheres obtained by in situ X-ray diffraction at an X-ray free-electron laser,” wrote Kraus.

The x-ray patterns showed how carbon moved step by step, first forming diamond, then mixing with liquid, and finally melting completely.

This helped scientists pinpoint the melting point in the range of 100 to 160 gigapascals, which is equal to roughly 1 to 1.6 million times the pressure at sea level.

Liquid carbon in the real world

Planetary experiments have shown that carbon can crystallize into diamonds under ice giant conditions. Independent laser experiments reproduced the high pressure chemistry that leads to diamond formation inside Uranus and Neptune.

The new liquid-state structure fills a separate gap. It informs how carbon behaves before and after those diamonds appear, and how heat and pressure move the system across phases.

Fusion science benefits as well. Capsules used at high energy laser facilities rely on precise carbon behavior when a shock wave first compresses the target.

In 2022, the National Ignition Facility reported ignition, meaning a fusion shot produced more energy than the lasers delivered to the target. Better knowledge of liquid carbon helps refine the compression stage that sets up those shots.

Peer-reviewed results

The researchers used a direct path from measured patterns to physical structure. They derived S(k), computed g(r), and extracted coordination numbers from the peaks without assuming a simple liquid model.

They then cross checked with ab initio simulations. The agreement ruled out oversimplified models that would predict many more neighbors than observed.

They also exploited mixed phase states carefully. Those conditions allowed them to deduce differences in density between diamond and liquid and to estimate the melting slope.

The timing precision was crucial. Ultrafast x-ray pulses froze motion on the relevant time scale and avoided blurring from rapid changes in pressure and temperature.

Liquid carbon implications

The same approach can extend to other light elements at extreme conditions. That means cleaner inputs for planetary interiors, shock physics, and high energy density materials design.

Higher repetition and faster analysis will raise precision. Facilities report that second scale data streams are possible as automation improves, which will push uncertainty down further.

The study is published in Nature.

Image and illustration credits: European XFEL/Jan Hosan and Martin Kuensting / HZDR).

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