Direct measurements of diamond melting under extreme pressure resolved a long-standing theory-experiment mismatch and could improve both fusion research and planetary models.
Diamond is not only a gemstone. This exceptionally hard form of carbon is also used in the tiny capsules that surround fuel in inertial confinement fusion experiments, and researchers think carbon may form diamonds that fall through the interiors of ice giants such as Neptune and Uranus.
Both environments expose diamond to immense pressure, yet experiments and computer simulations have long produced conflicting descriptions of what happens to the material under such extreme conditions.
Researchers at Lawrence Livermore National Laboratory (LLNL) have now measured diamond as it melts at pressures reaching three times those found at Earth’s core. Their findings were published in Nature Physics.
“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 author and LLNL scientist Marius Millot.
The measurements resolve two persistent disagreements in the field and bring experimental results into close alignment with simulations based on quantum mechanics. The findings could have consequences for inertial confinement fusion, where predictions suggest they may enable up to three times greater energy gain, as well as for models describing the interiors of planets under extreme pressure.
A decades-old mismatch is resolved
LLNL researchers have spent decades investigating diamond under extreme conditions. About 20 years ago, lab scientist Jon Eggert and his colleagues carried out pioneering high-pressure melting experiments and found that diamond became denser when it melted.
“While this is rather unusual among most materials, we all know an example of such behavior,” said LLNL scientist Marius Millot. “Liquid water is denser than ice, which makes ice cubes float. Jon’s finding means that diamond would float in liquid carbon at high pressures.”
But those influential experiments also left behind a major puzzle. The experimentally measured melting temperature differed from theoretical predictions by roughly 20%.
“No matter what the theorists did—even with the most advanced computer simulation techniques—they could not reproduce the experiments,” said Millot.
Another unanswered question emerged from experiments at Sandia National Laboratories. Researchers there used the powerful magnetic fields of the Z machine to shock compress small diamond samples and detected signals suggesting that diamond might transform into another crystalline form before melting. Simulations supported that possibility, but the atomic arrangement had never been measured directly.
X-rays capture diamond as it melts
To resolve both problems, LLNL researchers performed laser-driven dynamic compression experiments at the University of Rochester’s Laboratory for Laser Energetics (LLE). At the Omega Laser Facility, lasers vaporized the outer surface of a tiny diamond sample, generating a powerful shockwave that compressed the material inside.
Obtaining precise measurements during this process was particularly difficult because the extreme pressure states lasted only about one billionth of a second. Within that brief interval, the researchers needed to collect multiple measurements, including X-ray diffraction, which reveals how atoms are arranged.
“This was the first time that shock-compressed diamond was probed with X-ray diffraction all the way up to melting,” said Millot. “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.”
Improved diagnostic instruments developed and maintained by the LLE team produced a new measurement of diamond’s melting temperature. This time, the experimental value agreed almost perfectly with simulations, resolving the discrepancy that had persisted for about two decades.
“While it was frustrating to discover that our original temperature measurements were off by more than 1,000 degrees, it is exciting to see such a dramatic improvement in data quality with our new diagnostics,” said Eggert. “Even better, our original inference of melting has now been confirmed directly with X-ray diffraction.”
The experiment also produced a different result from the possible phase transition previously indicated at Sandia. Under a single shock, the carbon retained its diamond structure until it melted, with no intermediate crystalline phase detected.
“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,” said Millot.
That distinction may matter for future high-energy density experiments and simulations because it indicates that a material’s response depends not only on its pressure and temperature but also on the way the shock is delivered.
Slower shocks could boost fusion yield
The agreement between theory and experiment may have direct consequences for fusion experiments.
In inertial confinement fusion, powerful lasers generate shocks that force a tiny diamond capsule inward. The resulting implosion compresses the fusion fuel enclosed within it until the fuel reaches the extreme pressures and temperatures required for fusion.
During the first shock, the diamond needs to melt into a smooth, uniform fluid. Irregularities in the implosion can otherwise interfere with the fusion reaction. To ensure that melting occurs, scientists at LLNL’s National Ignition Facility (NIF) generally begin with a relatively strong shock.
“Our work indicates that we could use slightly slower initial shocks and still achieve full melting of the diamond in our NIF implosions,” said Millot. “This is exciting because such a slower shock would make the fusion fuel more compressible. That in turn increases the maximum energy yield we could obtain with the same laser energy.”
If other sources of performance loss can be controlled, predictions indicate that using these slower shocks could triple the energy gain.
The findings reach inside ice giants
The new measurements also provide useful information for scientists trying to understand Neptune and Uranus. Because the deep interiors of these ice giants cannot be observed directly, planetary researchers rely on experiments and models to determine what may occur far beneath their surfaces. Some studies suggest that carbon can crystallize at depth and fall through the planets as “diamond rain.” Since the latest experiments reached pressures greater than those expected inside ice giants, the revised melting data can help support more realistic models of their formation and evolution.
LLNL researchers next plan to use the capabilities of NIF to investigate diamond under even more extreme conditions that are difficult to reproduce elsewhere. They hope to determine how diamond capsules behave during later stages of an implosion and establish how long the diamond structure can remain stable when subjected to a sequence of multiple shock waves.
Reference: “Diamond melting in shock compression experiments at 1 TPa pressures” by Marius Millot, Federica Coppari, Amy Lazicki, Yong-Jae Kim, Otto L. Landen, Vladimir A. Smalyuk, Peter M. Celliers and Jon H. Eggert, 13 August 2026, Nature Physics.
DOI: 10.1038/s41567-026-03413-1
This study was supported by LLNL’s Laboratory Directed Research and Development program.
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