Diamond is famous for its beauty, but its value extends far beyond jewelry. This exceptionally hard form of carbon is used to make the tiny capsules that hold fuel in inertial confinement fusion experiments. Scientists also think diamonds may form and fall like rain far beneath the surfaces of ice giant planets such as Neptune and Uranus.
In both environments, diamond is subjected to immense pressure. Yet researchers have struggled for years to determine exactly how the material responds under such extreme conditions because laboratory measurements and computer simulations have produced conflicting results.
A new study published in Nature Physics may finally resolve that problem. Researchers at Lawrence Livermore National Laboratory (LLNL) measured how diamond melts at pressures three times greater than those found at Earth’s core.
“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 results settle two long-standing discrepancies in diamond research and bring experimental measurements into close agreement with simulations based on quantum mechanics. The findings could also have important practical consequences. Applying them to inertial confinement fusion may allow researchers to triple energy gain, while a clearer picture of diamond’s behavior at high pressure could improve models of planetary interiors.
A 20 Year Diamond Melting Mystery
LLNL researchers have investigated diamond under extreme conditions for decades. About 20 years ago, laboratory scientist Jon Eggert and his colleagues carried out pioneering experiments on diamond melting at high pressure. Their work produced an unusual observation: 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.”
Although the experiments marked an important advance, they also created a major puzzle. The melting temperatures measured in the laboratory differed by roughly 20% from temperatures predicted by theoretical models.
“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 tiny diamond samples. Their measurements produced signals suggesting that diamond might pass through another crystalline structure before melting completely into liquid carbon.
Computer simulations supported that interpretation. However, researchers had not been able to directly observe the atomic structure of the compressed material, leaving the proposed intermediate phase unconfirmed.
Lasers Reveal What Happens as Diamond Melts
To investigate both mysteries, the LLNL team performed laser-driven dynamic compression experiments at the University of Rochester’s Laboratory for Laser Energetics (LLE).
At the Omega Laser Facility, researchers used intense laser energy to vaporize the outer layer of a tiny sample. That process launched a powerful squeezing shockwave through the diamond inside.
Collecting precise measurements was especially difficult because the extreme pressure conditions lasted for only about a billionth of a second. During that brief interval, the researchers needed to record several properties of the material, including X-ray diffraction measurements capable of revealing its atomic arrangement.
“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.”
Researchers at LLE helped develop and maintain the improved diagnostic equipment that made those measurements possible. Using the new tools, the team obtained an updated melting temperature that matched computer simulations almost perfectly, resolving a disagreement that had persisted for roughly 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.”
Diamond Stays Diamond Until It Melts
The experiments resolved the melting temperature discrepancy, but they produced a different answer to the question raised by the Sandia results.
Instead of transforming into another crystalline phase before melting, the carbon kept its diamond structure all the way to the liquid state. No intermediate phase appeared during the experiment.
“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 could matter for future experiments and simulations involving materials at extremely high energy densities. The results suggest that the way a shock is delivered can influence how a material responds. Pressure and temperature alone may not determine which structure the material adopts.
Diamond Physics Could Improve Fusion Energy
The new agreement between theory and experiment has immediate relevance for inertial confinement fusion research.
In these experiments, powerful lasers create shock waves that force a tiny diamond capsule to implode. The collapsing capsule compresses fusion fuel to the extraordinary pressures and temperatures required for fusion reactions.
One important goal during the initial shock is to melt the diamond into a smooth, uniform fluid. Irregularities in the implosion can interfere with the compression process and weaken the resulting fusion reaction.
To ensure that the diamond capsule melts completely, scientists at LLNL’s National Ignition Facility (NIF) typically use a relatively strong initial shock.
The new measurements indicate that this first shock may not need to be as strong as previously thought.
“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.”
Models predict that using these slower shocks could potentially triple fusion energy gain, as long as researchers can keep other processes that reduce performance under control.
New Clues About Diamond Rain Inside Ice Giants
The findings could also help scientists understand what happens deep inside Neptune and Uranus.
Researchers cannot directly observe the interiors of these ice giants, so they depend heavily on laboratory experiments and computer models to reconstruct the extreme conditions beneath their surfaces.
Some studies suggest that carbon may crystallize into diamonds deep inside these planets. Those diamonds could then sink through the interior, creating what scientists describe as “diamond rain.”
Because the new experiments examined diamond at pressures even greater than those expected inside ice giants, the improved melting measurements give planetary scientists a stronger foundation for modeling how Neptune and Uranus formed and how their interiors have evolved.
Pushing Diamond to Even Greater Extremes
The LLNL team now plans to use the experimental capabilities of NIF to explore diamond under conditions that are even more difficult to reproduce.
Future work will examine how diamond capsules behave during later stages of a fusion implosion and investigate how long the diamond crystal structure can remain stable when the material is subjected to a sequence of multiple shock waves.
Other LLNL authors include Federica Coppari, Amy Lazicki, Yong-Jae Kim, Otto Landen, Vladimir Smalyuk and Peter Celliers. LLNL’s target fabrication specialist Renee Posadas and Eric Folsom at the HED Science Center Technology Facility also contributed to the work. This study was supported by LLNL’s Laboratory Directed Research and Development program.
