Diamond melting measured at record pressures, resolving 20-year discrepancy

Researchers at Lawrence Livermore National Laboratory have measured how diamond melts at pressures exceeding those inside Neptune and Uranus, resolving a two-decade conflict between laboratory data and quantum-mechanical simulations. The experiments, conducted at the University of Rochester's Omega Laser Facility, used intense laser pulses to shock-compress microscopic diamond samples to temperatures hotter than the Sun's surface and pressures above one terapascal.

For about 20 years, earlier melting-temperature measurements had differed from theoretical predictions by roughly 20 percent, and no simulation could reproduce the experimental results. The new study, published in Nature Physics, obtained X-ray diffraction data during the brief shock state, providing direct evidence of the diamond's atomic structure up to the melting point. The updated melting temperature now matches computer simulations almost perfectly.

The experiments also addressed a separate puzzle from Sandia National Laboratories' Z-machine studies, which had suggested diamond might transform into an intermediate crystalline phase before melting. The LLNL team found that under single-shock compression, diamond retains its cubic structure all the way to the liquid state, with no intermediate phase observed. The researchers attribute this to the extremely short timescale of the shock, which may trap the material in its original structure.

The findings have direct implications for inertial confinement fusion at the National Ignition Facility, where diamond capsules hold fusion fuel. The new melting data indicate that the initial shock used to melt the capsule can be slightly weaker than previously thought. A slower initial shock would make the fuel more compressible, and models predict this could potentially triple the fusion energy gain if other performance losses are controlled.

Beyond fusion, the results improve the foundation for modeling the interiors of ice-giant planets. Some theories propose that carbon crystallizes into diamonds deep inside Neptune and Uranus, which then sink as "diamond rain." Because the experiments reached pressures greater than those expected in those planets, the refined melting curve gives planetary scientists a more reliable benchmark for interior models.

The LLNL team plans to use the National Ignition Facility to study diamond under even more extreme conditions, including the response to multiple sequential shocks that occur during later stages of a fusion implosion. The work was supported by LLNL's Laboratory Directed Research and Development program.