The Hardest Mineral Has a Melting Point
Diamond is the hardest natural mineral on Earth, a symbol of eternity and the cornerstone of a multi-billion-dollar industry. But even its legendary toughness has limits. A new study, published in Nature Physics, has finally captured the detailed process of diamond melting under extreme conditions, putting an end to a two-decade debate between experimentalists and theoretical simulations.
Led by physicist Marius Millot at the Lawrence Livermore National Laboratory (LLNL) in California, the team compressed microscopic diamond samples using shock waves generated by the OMEGA laser system at the University of Rochester. Pressures reached between 600 gigapascals (GPa) and 1.8 terapascals (TPa)—more than three times the pressure at Earth's core—and temperatures exceeded those found on the Sun's surface.
The Mystery: A Liquid Denser Than a Solid
The controversy traces back to the early 2000s, when pioneering experiments by Jon Eggert and colleagues at LLNL revealed something extraordinary: under these extreme pressures, liquid carbon produced by melting diamond is denser than solid diamond, akin to ice floating on water. However, the melting temperatures measured in the lab did not match the most advanced quantum simulations, differing by about 20%.
Additionally, simulations had suggested the existence of a mysterious phase called BC8—a hypothetical crystalline form even harder than diamond—that could appear just before melting. But no one had ever observed it directly.
The Experiment: Lasers in a Billionth of a Second
To solve both puzzles simultaneously, Millot's team used lasers to vaporize the outer layers of diamond samples, creating shock waves that compressed them to the required pressure range. The feat was to measure what happened within just one billionth of a second. In that incredibly short window, they had to capture X-ray diffraction, density, temperature, and optical reflectivity data simultaneously.
The difficulty was immense, as Millot told Science Alert: 'Carbon is a small, light atom, which scatters very little X-ray signal.' Yet the measurement was a complete success. The new data placed the melting temperature at about 7,300 Kelvin (K) at a pressure near one terapascal—more than 1,000 K below previous experimental estimates and now perfectly matching quantum theory.
No BC8 Phase: The 'Trapped' Atom Surprise
The second mystery was resolved just as decisively: during compression by a single shock, the experiments detected no significant BC8 phase. X-ray diffraction still showed crystalline material, but the atoms kept the diamond structure. Millot explains: 'The sample doesn't have time to reorganize and remains trapped in the diamond structure.' In other words, melting occurs so rapidly that the intermediate BC8 phase never gets a chance to form.
This implies that BC8 is not a necessary step in diamond melting under these conditions, eliminating a hypothesis that had been on the table for years.
Why It Matters: Fusion Energy and Ice Giants
The implications reach far beyond the laboratory. First, in fusion energy research, the National Ignition Facility (NIF) at LLNL uses tiny diamond capsules to hold fuel that lasers compress in pursuit of nuclear fusion. Previously, engineers used a relatively strong first shock to ensure the diamond melted completely. With the new melting map, they could use a slower first shock without compromising full melting, keeping the fuel more compressible and potentially leading to a denser implosion. The researchers' calculations suggest this adjustment could triple the energy gain in fusion experiments, assuming other loss sources are controlled.
Secondly, the data also offer better constraints for models of the interiors of Uranus and Neptune. Since the 1980s, scientists have speculated that carbon might crystallize into diamond in the depths of these ice giants and precipitate downward—a phenomenon known as 'diamond rain'. While the experiments reach higher pressures than expected inside those planets, the new data will help refine when diamond remains stable or melts under planetary conditions.
Key Data from the Discovery
- Publication: Nature Physics
- Institution: Lawrence Livermore National Laboratory (LLNL)
- Pressures: 600 GPa to 1.8 TPa
- Event duration: 1 billionth of a second
- Melting temperature measured: 7,300 K (about 1,000 K lower than earlier estimates)
- BC8 phase: not detected