Researchers at Lawrence Livermore National Laboratory have published findings in Nature Physics that clarify how diamond responds to extreme pressure conditions, settling conflicting results that have persisted in the field for approximately 20 years. The new measurements reconcile discrepancies between laboratory experiments and theoretical computer simulations, bringing the two approaches into close alignment.
The investigation focused on two long-standing puzzles in diamond research. The first involved disagreement over the temperature at which diamond melts under extreme pressure, with laboratory measurements and theoretical predictions differing by roughly 20 percent. The second question centered on whether diamond undergoes a transformation into another crystalline structure before fully melting into liquid carbon, an interpretation that earlier Sandia National Laboratories experiments had suggested but could not directly confirm. Using laser-driven dynamic compression experiments at the University of Rochester’s Laboratory for Laser Energetics, the LLNL team applied intense laser energy to vaporize the outer layer of tiny diamond samples, creating powerful shockwaves through the material. The researchers employed X-ray diffraction measurements capable of revealing atomic structure, a particularly challenging measurement given that carbon scatters relatively few X-rays.
The new data revealed that the melting temperature was approximately 1,000 degrees higher than originally measured, now matching computer simulations nearly perfectly. Additionally, the experiments demonstrated that diamond maintains its crystalline structure all the way to the point of melting rather than transforming into an intermediate phase. Researchers attributed this to the short duration of the shock, which does not allow sufficient time for structural transformation. The distinction holds relevance for understanding how shock delivery methods influence material behavior beyond pressure and temperature alone.
The findings carry practical implications for inertial confinement fusion research, where diamond capsules are compressed by laser-driven shockwaves to achieve the extreme conditions necessary for fusion reactions. The revised understanding of diamond’s melting behavior suggests that initial shocks could be moderated while still achieving complete melting of the capsule. Such adjustment could improve the compressibility of fusion fuel and potentially triple energy gain in these experiments, while also enhancing models used to understand planetary interiors.
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