Scientists Melt Diamond At Record Pressures, Solving A Decades-Old Mystery About How It Behaves

A team of physicists has resolved a long-standing scientific puzzle over exactly how diamond melts under extreme pressure, using powerful laser-driven shock waves to observe the process in unprecedented detail and uncovering behavior researchers say defied expectations.

The research, led by physicist Marius Millot at Lawrence Livermore National Laboratory, was published this week in the journal Nature Physics. The team used the OMEGA Laser Facility at the University of Rochester’s Laboratory for Laser Energetics to blast tiny diamond samples with laser pulses, generating shock waves that produced pressures ranging from 600 gigapascals to 1.8 terapascals, more than three times the pressure found at the center of the Earth.

Diamond, the hardest naturally occurring mineral on Earth, is notoriously difficult to melt, requiring conditions far beyond anything found naturally at the planet’s surface. Scientists first successfully melted diamond in a 2007 experiment using a laser to strike it with a shock wave at 750 gigapascals of pressure, more than double the ambient pressure at Earth’s inner core. A follow-up study in 2009, led by LLNL physicist Jon Eggert, directly measured diamond’s melting temperature for the first time, but that measurement created a new problem: it disagreed with quantum-mechanical theoretical predictions by roughly 20%, a gap that persisted despite years of increasingly sophisticated computer modeling.

Millot’s team set out to resolve that discrepancy using significantly improved diagnostic tools that allowed them to track diamond’s atomic structure in real time as it melted, something earlier experiments lacked the sensitivity to capture. Using X-ray diffraction, the team measured how the diamond’s crystal structure evolved as pressure increased, alongside measurements of shock speed, temperature, density and optical reflectivity, all captured within a window lasting only a billionth of a second.

The new measurements placed diamond’s melting temperature at approximately 7,300 Kelvin at 1 terapascal of pressure, a figure notably lower than earlier experimental results and one that, for the first time, closely matched theoretical predictions. Eggert, a co-author on the new study, acknowledged the significance of the correction. “It was frustrating to discover that our original temperature measurements were off,” Eggert said, adding that the improved data quality made possible by the team’s updated diagnostics ultimately made the correction worthwhile.

Beyond resolving the temperature discrepancy, the experiment answered a second long-standing question about diamond’s behavior under extreme pressure. Theoretical models had predicted that, at pressures around a terapascal, diamond’s rigid atomic lattice should temporarily shift into a different crystalline form of carbon known as BC8 before fully melting into a disordered liquid. Some earlier experiments had been interpreted as possible evidence of that transition occurring above roughly 900 gigapascals.

Millot’s team found no evidence of a substantial BC8 phase. Instead, the X-ray data showed that diamond’s crystal structure remained in its familiar cubic lattice for as long as any solid structure could be detected at all, with solid diamond and liquid carbon simply coexisting until the material had fully melted. Millot attributed the finding to the extremely rapid, single-shock nature of the experiment. “We think that is because the sample does not have time to change when it only experiences a single shock,” Millot said, explaining that the diamond effectively becomes trapped in its original structure rather than transforming into the intermediate BC8 phase theory had predicted.

The researchers say that outcome likely reflects the sheer energy required to break and rearrange diamond’s unusually strong carbon-carbon bonds, a process theory suggests would be needed to form the BC8 structure. Under the fleeting timescale of the shock-compression experiment, the diamond appears to melt before that structural rearrangement has any opportunity to take place.

The findings also revealed an unexpected parallel between diamond and ordinary water ice. Under the extreme pressures studied, solid diamond proved less dense than the liquid carbon it melts into, mirroring the way conventional ice is less dense than liquid water and floats on its surface. The researchers say this implies solid diamond could similarly float atop a pool of molten carbon under the right conditions, and it also produces a related counterintuitive effect: across part of diamond’s melting range, increasing pressure actually lowers the temperature required for it to melt.

Beyond its implications for fundamental physics, the research carries direct practical applications. Diamond is used to encase fuel pellets in inertial confinement fusion experiments, where the material is compressed under pressures similar to those studied in the new experiment. More accurate models of how carbon behaves as those capsules implode could improve the design of future fusion experiments. According to the study’s authors, simulations incorporating the corrected carbon behavior suggest that predicted fusion energy gain could increase by up to threefold. Millot said the findings suggest researchers could achieve full diamond melting in fusion experiments using somewhat slower initial shocks, a change that would make the fusion fuel more compressible and could increase the maximum energy yield achievable using the same amount of laser energy.

The research also carries implications beyond Earth. Scientists believe carbon experiences pressure and temperature conditions similar to those recreated in the experiment deep inside ice giant planets such as Neptune and Uranus, meaning the improved understanding of diamond’s high-pressure behavior could help refine models of those planets’ internal structure and composition.

Separately, researchers at Sandia National Laboratories have conducted related diamond-compression experiments using the extreme magnetic fields generated by that facility’s Z machine, and have reported experimental signatures suggesting diamond may take an additional intermediate step on its way to melting, a finding that appears to differ from the Livermore team’s conclusions and suggests the full picture of diamond’s high-pressure behavior may still require further study to fully resolve.