Behold, liquid carbon
Carbon is famous for its many solid forms. It's the soot in air pollution, the graphite in pencil leads, and the glittering diamond in expensive jewellery. It's also the carbon nanotubes in biosensors and fullerenes in organic solar cells.
However, despite its ability to exist in various shapes as a solid, carbon's liquid form has been a long-standing mystery. The main reason is that carbon is very difficult to liquefy: at around 4,500º C of temperature and a hundred atmospheres' worth of pressure — not something found even inside a blast furnace. Scientists have thus struggled to see what molten carbon actually looks like.
The question of its structure isn't only scientific curiosity. Liquid carbon shows up in laser-fusion experiments and in the manufacture of nanodiamonds. The substance probably also exists deep inside planets like Uranus and Neptune.
In a 2020 review of the topic in Chemical Physics Letters, three researchers from the University of California Berkeley wrote:
[M]any intriguing unanswered questions remain regarding the properties of liquid carbon. While theory has produced a wide array of predictions regarding the structure, phase diagram, and electronic nature of the liquid, as of yet, few of these predictions have been experimentally tested, and several of the predicted properties of the liquid remain controversial.
In a major step forward, an international collaboration of researchers from China, Europe, the UK, and the UK recently reported that they had managed to briefly liquefy carbon — but long enough to observe the internal arrangement of its atoms in detail. They achieved the feat by blasting a carbon wafer with a powerful laser, then X-raying it in real time.
The researchers used glassy carbon, a hard form of carbon that absorbs laser energy evenly.
To create the extreme conditions required to liquefy, the team used the European XFEL (EuXFEL) research facility in Germany. Here, a power laser fired 515-nm light to the front of a glassy carbon wafer. The pulse lasted 5-10 nanoseconds, was roughly one-fourth of a millimetre wide, and carried up to 35 joules of energy.
That's just one-tenth of the energy required to melt 1 g of ice. But because it was delivered in concentrated fashion, the pulse launched a shockwave through the wafer. Shock compression simultaneously squeezed and heated the material, quickly driving pressures to 7 lakh to 16 lakh times the earth's atmosphere. The temperature in the wafer also soared above 6,000 K — well into the liquid-carbon regime.
Then, a device recorded the speed of the shockwaves and confirmed the wave stayed flat and steady across the region to be blasted by X-rays. With the wave speed and the sample's thickness, the team calculated the pressure inside the sample to about 98,000 atm.
While the shockwaves were still rippling through the sample, the EuXFEL facility launched a 25-femtosecond-long flash of X-rays at the same spot.
The liquid carbon state lasted for only a few nanoseconds but a femtosecond is a million-times even shorter.
The X-rays scattered off the carbon atoms and were caught by two large detectors, where the radiation produced patterns called diffraction rings. Each ring encoded the distances between atoms, like a fingerprint of the sample's internal structure.
Because the X-ray flash was intense, each flash revealed enough data to analyse the liquid's structure.
For added measure, the team also varied laser power and the wafer's thickness to collect data across a range of physical parameters. For example, the pressure varied from 1 atm (for reference) to 15 lakh atm. Each pressure level corresponded to a separate, single-shot X-ray measurement, so the whole dataset was assembled shot by shot.
At pressures of 7.5-8.2 lakh atm, the glassy carbon began turning into crystalline diamond. At 10-12 lakh atm, the signs in the data that were symptomatic of diamond weakened while broad humps characteristic of the liquid phase emerged and grew. The scientists interpreted this as evidence of a mixed state where solid diamond and liquid carbon coexist.
Then, at about 15 lakh atm, the data pertaining to the diamond form vanished completely, leaving only the broad liquid-form's humps. The sample was now fully molten. According to the team, this means carbon under shock melts roughly between roughly 9.8 lakh and 16 lakh atm in the experiment.
Then, to convert the diffraction patterns into information about the arrangement of atoms, the team used maths and simulations.
The team members calculated the static structure factor, a framework that described how the atoms in liquid carbon scattered the X-ray radiation. Then they used the factor to estimate the chance of finding another carbon atom at some distance from a reference atom. These distances indicated the distances the atoms preferred to keep and the average number of nearest neighbours.
Next, they used quantum density-functional theory molecular dynamics (DFT-MD) to simulate how 64 carbon atoms move at a chosen density and temperature. The simulations produced static structure factor data that the researchers compared directly to their data. By adjusting the density and temperature in the simulation, they found the best-fit values that matched each experiment.
The team performed this comparison because it could rule out models of liquid carbon's structure that were incompatible with the findings. For example, the Lennard-Jones model predicted the average number of neighbouring carbon atoms to be 11-12, contrary to the data.
The team estimated that carbon melted temperature to around 6,700 K and a pressure of 12 lakh atm. When fully molten, each carbon atom had about four immediate neighbours on average. This is reminiscent of the way carbon atoms are arranged in diamond, although the bonds in liquid carbon are also constantly breaking and reforming.
The near-perfect fit between experiment and DFT-MD for the structure factor indicated that existing quantum simulation techniques could capture liquid carbon's behaviour accurately at high pressure. The success will give researchers confidence when using the same methods to predict even harsher conditions, such as those inside giant exoplanets.
Indeed, ice-giants like Neptune may contain layers where methane breaks down and carbon forms a liquid‐like ocean. Knowing the density and the atomic arrangement in such a liquid can help predict the planet’s magnetic field and internal heat flow.
Similarly, in inertial-confinement fusion — of the type that recently breached the break-even barrier at a US facility — a thin diamond shell surrounds the fuel. Designers must know exactly how that shell melts during the first shocks to generate power more efficiently.
Many advanced carbon materials such as nanotubes and nanodiamonds form when liquid carbon cools rapidly. Understanding how the liquid's atoms establish short-range order could suggest pathways to tailor these materials.
Finally, the team wrote in its paper, the experiment showed that single-shot X-ray diffraction combined with a high-repetition laser can map the liquid structure of any light element at extreme pressure and temperature. Running both the laser and EuXFEL at their full capabilities could thus allow scientists to put together large datasets in minutes rather than weeks.
