Researchers at Lawrence Livermore National Laboratory used intense laser shock compression to measure diamond melting at pressures three times greater than Earth’s core. Published in Nature Physics, the experiments resolved a 20-year scientific discrepancy, aligning laboratory observations with quantum-mechanical simulations and offering new implications for inertial confinement fusion and planetary science.
Laser-Driven Shock Compression at the Omega Laser Facility
To investigate how carbon behaves under extreme forces, scientists performed laser-driven dynamic compression experiments at the University of Rochester’s Laboratory for Laser Energetics (LLE). Researchers utilized the Omega Laser Facility to vaporize the outer layer of a tiny carbon sample, launching a powerful squeezing shockwave directly through the diamond inside.
Capturing precise data during the procedure required overcoming severe experimental hurdles. During that brief window, the team recorded atomic structure, temperature, density, and optical reflectivity using advanced X-ray diffraction techniques.
Carbon atoms are small and lightweight, meaning they scatter very few X-rays and produce exceptionally faint signals. Enhanced diagnostic equipment developed and maintained by researchers at LLE made it possible to probe shock-compressed diamond with X-ray diffraction all the way up to its melting point.
Resolving a Two-Decade Melting Temperature Discrepancy
About 20 years ago, pioneering high-pressure melting experiments carried out by laboratory scientist Jon Eggert and his colleagues produced an unusual observation: diamond became denser when it melted. This behavior mirrors liquid water and ice, implying that solid diamond would float in liquid carbon at high pressures as detailed in the published findings.
However, those early laboratory findings sparked a persistent puzzle. The melting temperatures recorded by researchers differed by roughly 20% from predictions generated by advanced theoretical computer models based on quantum mechanics.
The updated measurements obtained during the recent LLE experiments yielded a melting temperature that matched computer simulations almost perfectly, closing the 20-year gap. 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,
Eggert noted in statements released by the research team. The work directly confirmed the original inference of melting using modern X-ray diffraction data.
Contrasting Results on Intermediate Phase Transitions
While the new dynamic compression experiments successfully solved the melting temperature mystery, they also challenged a separate hypothesis proposed by researchers at Sandia National Laboratories. Scientists at Sandia had previously utilized the powerful magnetic fields of the Z machine to shock compress small diamond samples, obtaining experimental fingerprints that suggested diamond might transition through an extra crystalline structure before melting completely into liquid carbon according to laboratory reports.
Computer simulations supported the presence of an intermediate phase, but researchers had lacked direct atomic-level observation to confirm it. The laser-driven experiments at Rochester told a different story: the carbon remained in a standard diamond structure all the way until it melted, skipping any intermediate phases entirely.
Scientists attribute this difference to the speed of the single shockwave. Because the sample experiences a single shock so rapidly, it does not have time to change its crystal lattice and remains trapped in the diamond structure as explained by LLNL researchers. This outcome indicates that material behavior under extreme stress depends heavily on the precise application path of the shock rather than solely on final pressure and temperature thresholds.
Implications for Planetary Interiors and Fusion Energy
Understanding how carbon responds under multi-megabar stresses carries direct applications for both astrophysics and energy research. Scientists believe diamonds form and fall like precipitation deep inside the interior mantles of ice giant planets such as Neptune and Uranus under extreme conditions. Refining experimental data on diamond melting allows geophysicists to build more accurate interior models for these distant planets.
At the same time, exceptionally hard carbon capsules are utilized to encase nuclear fuel pellets in inertial confinement fusion experiments pursued at national laboratories. Applying the new findings regarding diamond equation of state to fusion ablators could potentially help researchers triple energy gain in future high-energy-density experiments.
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