New laser experiments on tiny synthetic samples have revised the temperature at which diamond melts, finding a value more than 1,300°F (700°C) lower than earlier laboratory reports. The work, published in Nature Physics, used intense ultraviolet pulses to generate shock waves that made the material switch from transparent to mirror-like while also producing measurable thermal emission — two signals the team interprets as evidence of melting. Co-author Marius Millot and collaborators combined those optical signatures with X-ray diffraction to map melting under pressures and temperatures previously hard to probe.
The new measurements close a puzzling discrepancy: prior experimental data placed the melting point roughly 2,240°F (1,244°C) higher than theoretical model predictions. In the recent experiments the melting temperatures align with model expectations, removing the nearly 20% disagreement that had persisted for two decades. The researchers report being able to compress and heat samples to conditions hotter than the Sun’s surface and to pressures exceeding those at the centers of Uranus and Neptune while still recording atomic structure and optical properties.
X-ray diffraction during shock loading showed no intermediate transition to another solid form of carbon before liquefaction, suggesting the energy barrier for a solid-solid rearrangement is too large under the applied single shocks. The team notes, however, that different shock sequences might permit such a transition. They measured a regime — between about 660 and 1,060 gigapascals and at temperatures near 12,140°F (6,727°C) — in which solid diamond could coexist as pieces suspended in a metallic liquid carbon, a phase that is denser and electrically conductive.
The findings carry practical significance for nuclear fusion research and for models of planetary interiors. Fusion experiments sometimes employ diamond-based capsules that are compressed and heated by lasers; accurate melting data affect predictions of capsule behavior under extreme loading. The results also refine expectations for the mantles of ice giants such as Uranus and Neptune, where liquid carbon and diamond phases may shape heat transport and carbon cycling deep below the surface. Lawrence Livermore National Laboratory researchers are among the authors leading this experimental advance, which narrows a key gap between theory and laboratory observation.
