New melting experiments confirm that diamond floats in metallic liquid carbon at high pressures, much like ice cubes float in a glass of water.
Diamond remained recognizably diamond — the same familiar cubic arrangement of carbon atoms — until its crystal lattice simply disappeared into liquid.
The measurements, reported in Nature Physics, also settle a roughly 20-year disagreement over diamond’s melting temperature.
That produces an almost water-like oddity: in this pressure range, liquid carbon is denser than solid diamond.
The new experiments suggest the problem with the earlier diamond melting point estimates was not with the theory.
New melting experiments confirm that diamond floats in metallic liquid carbon at high pressures, much like ice cubes float in a glass of water. Credit: James Wickboldt/LLNL.
Diamond has a reputation for being indestructible. It’s the hardest material in the natural world. But hit a tiny piece with pressures millions of times greater than the atmosphere and temperatures hotter than the Sun’s surface, and even diamond melts.
The surprise is what it doesn’t do first.
In experiments at the Omega Laser Facility in Rochester, N.Y., physicists blasted synthetic diamonds with powerful lasers and watched their atomic structure as pressures climbed toward a trillion pascals, or one terapascal. Diamond remained recognizably diamond — the same familiar cubic arrangement of carbon atoms — until its crystal lattice simply disappeared into liquid. The researchers found no substantial intermediate crystal phase that theory says should become more stable at such pressures. It’s as if an ice cube turned into slush almost instantly without any transition between the phases.
The measurements, reported in Nature Physics, also settle a roughly 20-year disagreement over diamond’s melting temperature. The researchers put the melting temperature at about 7,300 kelvin — roughly 7,000 degrees Celsius, or 12,700 degrees Fahrenheit — at a pressure of around one terapascal, nearly 10 million times atmospheric pressure at sea level. That is more than 1,000 degrees Celsius lower than an influential earlier experimental estimate, and much closer to what modern computer simulations had predicted.
“We were able to take tiny diamond samples and shock compress them to temperatures hotter than the surface of the sun and to pressures higher than the center of Neptune and Uranus — and still measure atomic structure, temperature, density and optical reflectivity,” said Marius Millot, a physicist at Lawrence Livermore National Laboratory (LLNL) and the study’s lead author.
A 20-year Disagreement, Answered in a Billionth of a Second
Scientists have melted diamond before. A landmark Nature Physics experiment published in 2009 showed that under enormous pressure it melts into an unusually dense, electrically conducting carbon fluid. The resulting liquid carbon was denser than the solid.
But one part of that experiment remained puzzling. The researchers also estimated the temperature at which the diamond melted, and that number did not match increasingly sophisticated computer simulations of carbon. The gap was large — about 1,500 kelvin, equivalent to a difference of about 1,500 degrees Celsius or 2,700 degrees Fahrenheit, as revealed by this new study.
For years, theorists refined their calculations, yet the mismatch persisted.
To find out why, Millot’s team ran 12 experiments with nearly steady shock waves and seven with shocks that weakened as they crossed the diamond. Laser pulses generated pressures from roughly 600 to 1,800 gigapascals. The compressed state survived only nanoseconds. During that instant, instruments tracked the shock’s speed, the diamond’s brightness and reflectivity, while X-rays recorded its atomic structure.
“This was the first time that shock-compressed diamond was probed with X-ray diffraction all the way up to melting,” Millot said. “These measurements are extremely difficult because carbon is a small and lightweight atom. It scatters very few X-rays, so the signal we needed to measure was quite faint.”
Between roughly 750 and 1,000 gigapascals, the measured temperature actually slipped downward as pressure increased, hovering around 7,200 to 7,400 kelvin. Meanwhile, reflectivity rose sharply as more metallic liquid carbon appeared.
That produces an almost water-like oddity: in this pressure range, liquid carbon is denser than solid diamond. In principle, a solid diamond crystal could therefore float on molten carbon, much as ice floats on water.
The new experiments suggest the problem with the earlier diamond melting point estimates was not with the theory. With improved temperature measurements, the researchers found that diamond melts at a substantially lower temperature than the earlier experiment had indicated, bringing the laboratory results into close agreement with the simulations.
Diamond Refuses to Become BC8
The second mystery concerned a hypothetical high-pressure form of carbon called BC8.
Calculations predict that BC8 should eventually become more stable than ordinary cubic diamond. Earlier shock experiments had been interpreted as evidence that this transformation begins around 900 gigapascals.
The new X-ray measurements saw something else.
As melting progressed, the diffraction signal from crystalline diamond fell roughly tenfold and approached zero. But as long as a crystal signal remained, its density and diffraction pattern matched ordinary diamond. Assigning the signal to BC8 produced densities about 30 percent away from previous measurements, and researchers saw none of the additional diffraction lines expected from a substantial BC8 phase.
“We think that is because the sample does not have time to change when it only experiences a single shock. It remains ‘trapped’ in the diamond structure,” Millot said.
Previously, in a 2021 Nature experiment, researchers compressed diamond to pressures as high as two terapascals without seeing it transform into another crystal structure. That is essentially what Millot and his colleagues now see during shock compression: the diamond remains trapped in its familiar cubic structure until melting begins.
A 2023 Physical Review Letters study then offered a possible explanation for how the elusive BC8 form might nevertheless be produced. Its simulations suggested that one sudden shock may not give the carbon atoms enough time or the right pathway to rearrange themselves. Instead, a carefully timed second shock could push the material along a different route into BC8. The new experiment did not find BC8 during a single, nanosecond-scale shock, suggesting that pressure and temperature alone do not determine carbon’s fate. The route taken to reach those conditions — and how much time the atoms have to reorganize — may matter just as much.
From Diamond Rain to Fusion Power
At the National Ignition Facility, fusion fuel sits inside a tiny, extremely smooth diamond capsule. Lasers drive shocks through that shell, sending it inward at more than 400 kilometers per second. Engineers currently use a relatively strong first shock — above 1.2 terapascals — partly to make sure the diamond melts completely and evenly.
The revised melting curve suggests that a weaker first shock could still do the job. That would keep the fuel more compressible, potentially allowing a denser final implosion. The authors’ modeling suggests that, if other sources of performance loss can also be controlled, the change could eventually produce as much as three times the fusion energy of current designs.
There are also potential implications in astronomy and planetary science. Since at least the 1980s, researchers have proposed that the extreme pressures inside Uranus and Neptune could force carbon-rich material to separate and crystallize into diamond. Those diamonds might then sink through the planets’ interiors — the process often described as “diamond rain.”
Uranus and Neptune contain complex mixtures of water, methane, ammonia and other materials, whereas these experiments examined diamond itself. But the researchers recreated pressures that reach beyond those expected inside the ice giants and measured exactly when diamond remains solid, begins to melt and turns into liquid carbon. Those measurements give planetary scientists better constraints for models of where diamond could remain stable inside an ice giant, where it might melt, and how carbon may move through the planet’s deep interior.
