They Crushed Diamond Past the Pressure Inside Neptune and Watched It Melt. A 20% Gap Between Theory and Experiment Just Closed.

For twenty years, one of the cleanest tests in high-pressure physics kept coming out wrong. Laboratories that squeezed diamond until it melted got a melting temperature roughly 20 percent away from what quantum-mechanical simulations said it should be. Theorists rebuilt the calculations repeatedly and could not close the gap. "No matter what the theorists did," said Marius Millot of Lawrence Livermore National Laboratory, "they could not reproduce the experiments."

A paper published in Nature Physics this month reports that the gap is gone. A Livermore-led team including Millot, Jon Eggert, Federica Coppari and Amy Lazicki, working with the University of Rochester's Laboratory for Laser Energetics, drove shock waves through small diamond samples at the Omega Laser Facility and took measurements while the material was compressed — a window that lasts a few billionths of a second.

The conditions were extreme even by the standards of the field. The team pushed diamond past 1 terapascal — about a trillion pascals, roughly three times the pressure at the center of the Earth — and to temperatures above the surface of the Sun. Those are the conditions found in the deep interiors of Neptune and Uranus. Using X-ray diffraction fired through the sample during the nanosecond of compression, the researchers read out the atomic structure directly, along with temperature, density and optical reflectivity.

What they found is that diamond holds its crystal structure right up to the point where it melts into liquid carbon, with no intermediate phase in between. That absence is what had been distorting the earlier picture. Once the measurements were made cleanly, they lined up almost exactly with simulations built from first-principles quantum mechanics. The work also confirmed an earlier and counterintuitive result of Eggert's: at these pressures diamond becomes denser when it melts, which means solid diamond floats in its own liquid the way ice floats in water. Classical intuition says the opposite.

The practical payoff is in inertial confinement fusion, where fuel capsules are made of diamond and are supposed to be crushed symmetrically by laser-driven shocks. Designers have been using melting curves that were 20 percent off. With accurate numbers, the initial shock can be made gentler while still fully melting the capsule, which leaves the fuel more compressible at the moment of implosion. The team's analysis suggests that change alone could roughly triple the energy gain of a shot — a large step for a program whose entire record of net-energy results dates to the past few years.

The other payoff is planetary. Ice giants like Neptune and Uranus are thought to rain diamonds: carbon compounds deep in their mantles separate under pressure, and the diamond sinks. Every model of that process depends on knowing where the melting line sits and whether the solid floats or falls. Getting the density inversion right changes the direction the diamond travels, which changes how heat and material move through a planet's interior — and that in turn changes what the models predict about how ice giants form, cool and generate magnetic fields.

None of this required a new facility. It required measuring atomic structure, temperature and density simultaneously during an event that lasts less than the time it takes light to cross a room, and doing it well enough that theory had nowhere left to hide.