Scientists have revised the diamond melting point after laser shock experiments showed it is more than 1,300 degrees Fahrenheit lower than earlier measurements, resolving a long-standing gap between data and theory.
Although diamond is the hardest natural material on Earth, it melts under extreme conditions. Understanding its behavior under powerful laser-driven shock waves is central to nuclear fusion research. Fusion relies on precise models of how materials respond at very high temperatures and pressures, so physicists have long sought to pin down the melting behavior of diamond with confidence.
For years, models and experiments diverged. Earlier data suggested melting temperatures about 2,240 F, or 1,244 C, higher than theoretical predictions. Researchers also debated whether diamond transforms into another solid form of carbon before becoming liquid. The difficulty of reproducing and measuring such extreme states in the lab left the discrepancies unresolved.
Diamond melting point aligned with theory
In work published Aug. 13 in Nature Physics, a team used an ultraviolet laser to drive intense shock waves through tiny plates of synthetic diamond. As the shock passed, the samples shifted from transparent to mirror-like, a sharp rise in reflectivity that indicated melting. Combining optical reflectivity with self-emission measurements let the team chart the melting temperature with high precision.
“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 co-author Marius Millot of Lawrence Livermore National Laboratory in a statement.
The results show the melting temperature is more than 1,300 F lower than previously reported, bringing the diamond melting point into agreement with theoretical expectations and closing a roughly 20% gap.
X-ray diffraction measurements further revealed no intermediate transition to another solid carbon phase before melting, which the authors suggest could be because the energy barrier for atomic rearrangement is too high under single-shock conditions. They hypothesize that multiple shocks, or different shock profiles, might still induce such a phase change, a question that matters for fusion experiments where lasers implode diamond capsules containing deuterium and tritium to reach pressures above 30 petapascals and temperatures exceeding 100 million C.
Solid diamond in metallic liquid carbon
The team mapped a regime between roughly 660 and 1,060 gigapascals and around 12,140 F, or 6,727 C, where diamond coexists as solid chunks within liquid carbon. As pressure rises, more of the solid converts to liquid. Unlike graphite, coal, or diamond, liquid carbon in this state is metallic, conducting electricity and exhibiting a density higher than that of solid diamond. In principle, a diamond fragment could float in this metallic liquid like an ice cube in water.
These findings also inform models of Uranus and Neptune. Based on spacecraft measurements and prior laboratory work, scientists think it may rain large diamond pieces inside these ice giants and that their mantles could harbor oceans of liquid carbon with diamond bergs. The refined melting map enables more accurate predictions of their interior structure and carbon cycles.
Citation: Millot, M., Coppari, F., Lazicki, A., Kim, Y., Landen, O. L., Smalyuk, V. A., Celliers, P. M., & Eggert, J. H. (2026). Diamond melting in shock compression experiments at 1 TPa pressures. Nature Physics. Nature Physics. Related research on extreme environments appears in work such as High-temperature transistor hits 1110 F, eyed for Venus probes.