Researchers at Lawrence Livermore National Laboratory have documented how diamonds melt under extreme pressure, solving a decades-long scientific mystery and potentially opening the way to tripling the energy gained in fusion experiments. In a study published in Nature Physics on August 13, 2026, scientists led by Marius Milot used impact compression techniques to melt diamond samples at pressures three times greater than conditions found in the Earth’s core.
Experiments have finally resolved a stubborn conflict: For about 20 years, diamond’s measured melting temperature differed by about 20% from what quantum mechanical simulations predicted. “No matter what theorists did, even with the most advanced computer simulation techniques, they couldn’t reproduce the experiments,” Milo said.
Solve the puzzle
To address this discrepancy, the LLNL team conducted laser-driven dynamic compression experiments at the University of Rochester’s Laser Energy Laboratory using the Omega Laser Facility. Scientists vaporized the outer layer of tiny diamond samples, sending shock waves rocketing through the interior at pressures between 600 gigapascals and 1.8 terapascals, conditions hotter than the surface of the Sun and exceeding the pressure at the centers of Neptune and Uranus.
The major innovation was measuring X-ray diffraction data throughout the melting period, a feat that had never been accomplished before. “These measurements are very difficult because carbon is a small, lightweight atom. It scatters very few X-rays, so the signal we needed to measure was very weak,” Melott explained. The new measurements put the melting temperature of diamond at around 7,300 K at 1 TPa, which is much lower than older experimental readings and consistent with theoretical predictions.
The experiments revealed another surprising result, which is that diamond does not transform into an intermediate crystalline stage called BC8 before melting, as some previous theories suggested. Instead, the carbon remains trapped in its familiar diamond crystalline structure all the way until it melts into liquid carbon. “We think this is because the sample doesn’t have time to change when it is subjected to just one shock. It remains ‘trapped’ in the diamond structure,” Melott said.
One of the most unexpected findings is that solid diamond is actually less dense than the liquid carbon in which it is fused, a property it shares with ice and water. This means that diamonds could theoretically float in molten carbon at such extreme pressures, an unusual behavior among most materials.
Implications of fusion energy
The findings have direct consequences for inertial fusion, an approach used at the National Ignition Facility. In these experiments, powerful lasers generate shock waves that push a tiny diamond capsule inward, compressing the fusion fuel to the extreme pressures and temperatures needed for nuclear reactions. Dissolving the diamond capsule into a uniform fluid during the initial shock is critical to minimizing implosion defects that can reduce fusion yield.
Current NIF protocols use relatively strong initial shocks to ensure the diamond is completely dissolved. But the new research suggests that scientists could use slightly slower initial shocks and still achieve complete dissolution. “This is exciting because such a slower shock would make the nuclear fusion fuel more compressible,” Melott said. “This in turn increases the maximum energy output we can get with the same laser energy.” Computer simulations including the updated carbon model predict that these slower shocks could triple the energy gain, provided other degradation mechanisms can be controlled.
The work also extends beyond integration. These findings provide planetary scientists with a stronger basis for understanding the interiors of icy giant planets like Neptune and Uranus, where carbon may be subjected to similarly intense pressures and can form “diamond rain” deep below the surface. The new melting data will help improve models of planetary formation and evolution on these distant worlds.
sources
- Lawrence Livermore National Laboratory – Official announcement and quotes from Marius Mellot and John Eggert on diamond melting experiments, pressure conditions, and fusion effects
- Nature physics – Peer-reviewed publication of the study “Dissolution of Diamond in Shock Compression Experiments at 1TPa” (August 2026)
- Science Alert – A detailed explanation of experimental methods, X-ray diffraction measurements, and the absence of the BC8 transition phase
- Phys.org – Report the resolution of the 20-year temperature discrepancy and fusion energy gain predictions
