Scientists Melt Diamond at Neptune-Like Pressures, Solving a 20-Year Mystery

Diamond is prized for more than jewelry: this exceptionally hard form of carbon is used to make the tiny capsules that hold fuel in inertial confinement fusion experiments, and scientists think diamonds may form and fall like rain deep beneath the surfaces of ice giant planets such as Neptune and Uranus. In both settings diamond is subjected to immense pressure, but laboratory measurements and computer simulations of how it behaves under such conditions have long produced conflicting results. A new study in Nature Physics by researchers at Lawrence Livermore National Laboratory (LLNL) may finally resolve the disagreement.

About 20 years ago, LLNL scientist Jon Eggert and colleagues found that diamond becomes denser when it melts -- unusual behavior shared by few other materials, though liquid water is a familiar example, since it is denser than ice. That discovery also created a puzzle: the melting temperatures measured in the lab differed by roughly 20% from what theoretical models predicted, a gap that persisted despite advanced computer simulations. Separately, experiments at Sandia National Laboratories, using the powerful magnetic fields of the Z machine, produced signals suggesting diamond might pass through another crystal structure before fully melting, though this was never directly confirmed.

To investigate both mysteries, the LLNL team performed laser-driven compression experiments at the University of Rochester's Laboratory for Laser Energetics, using the Omega Laser Facility to launch a shockwave through tiny diamond samples and, for the first time, probe the compressed diamond with X-ray diffraction all the way to melting. '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 LLNL scientist Marius Millot.

The new measurements matched computer simulations almost perfectly, resolving the two-decade-old melting-temperature discrepancy, and showed that the diamond kept its crystal structure all the way to the liquid state rather than passing through an intermediate phase, as the Sandia results had suggested. Applying the findings to inertial confinement fusion, where lasers compress diamond-capsuled fuel, could help triple energy gain in such experiments, the researchers said.