For most substances, squeezing a solid raises the temperature at which it melts. For diamond, in a narrow and extreme window of pressure, the opposite happens. New experiments at the University of Rochester’s OMEGA laser facility show that between roughly 400,000 and 1 million times atmospheric pressure, crushing diamond harder actually lowers its melting temperature. The work, published August 13 in Nature Physics by a team from Lawrence Livermore National Laboratory, also settles a long-running argument about whether diamond transforms into a different crystal before it melts: it does not, at least not along the path the experiments take.
Laser pulses lasting nanoseconds drove shock waves between 600 and 1,800 gigapascals, up to 1.8 terapascal, through thin wafers of synthetic microcrystalline diamond. Twelve experiments used steady shocks, holding the pressure constant long enough to take an X-ray diffraction snapshot of the compressed crystal, and seven used decaying shocks to watch the material’s response across a range of pressures in a single shot. Three diagnostics worked together: an interferometer measuring shock velocity and reflectivity, a fast optical pyrometer reading the shock-heated material’s temperature, and X-ray diffraction probing the crystal structure itself.
Reflectivity at the shock front climbed from below 1 percent near a shock velocity of 21 kilometers per second to about 40 percent above 24.5 kilometers per second, a signature that the compressed diamond was becoming an electrically conducting fluid. The shock temperature behaved in a way that is impossible for a simple solid: instead of rising steadily with shock strength, it drifted slowly downward, from about 7,400 K near 22 kilometers per second to about 7,200 K near 24 kilometers per second, before turning sharply upward. X-ray diffraction showed a tenfold drop in signal from the shocked crystal, trending to zero near 24.2 kilometers per second. Melting, in other words, began around 750 gigapascals and was complete near 1,000 gigapascals, at temperatures around 7,300 K.
The counterintuitive slope has a physical explanation rooted in density. Between roughly 0.4 and 1 terapascal, liquid carbon is predicted to be denser than solid diamond, because atoms in the liquid pack into higher coordination than the four bonds of diamond’s structure. When the melt is denser than the crystal, the melting curve tilts downward: the Clapeyron slope, the thermodynamic relation between pressure and melting temperature, turns negative. Squeezing harder favors the denser liquid, so less thermal energy is needed to melt the solid. The same logic explains why diamond’s melting line has a maximum near 0.4 terapascal, above which the slope inverts.
The X-ray data also address the puzzle of the BC8 phase, a body-centered cubic arrangement of carbon predicted by density functional theory to become thermodynamically stable above about 1 terapascal. A 2008 study of flyer-plate impacts had interpreted compressibility changes near 900 gigapascals as evidence that diamond transformed into BC8 along the shock path. The new diffraction patterns contradict that reading. The single observed diffraction line from the compressed crystal matches the strongest reflection of ordinary cubic diamond, giving densities within 4 percent of velocimetry measurements; reading the line as BC8 instead gives densities roughly 30 percent off the velocimetry values, which the authors call unrealistic. No additional lines from BC8 appeared. The conclusion: cubic diamond persists all the way up to its melting point, holding a structure that theory says should be unstable, because the strong covalent bonds of diamond raise an enormous kinetic barrier against rearrangement. The result aligns with 2021 ramp-compression experiments that found diamond intact at 2 terapascal, and with machine-learning simulations of shock compression.
There is a correction in the paper as well. The pioneering 2010 decaying-shock measurement of diamond’s melting temperature, also from the Livermore group, ran over a thousand kelvin hotter than the new data. The authors attribute the gap to experimental design: the new campaign calibrates the pyrometer against a quartz reference plate and relies on upgraded cameras and optics. The new temperatures also close a roughly 1,500 K discrepancy that had separated experiment from quantum simulation, providing the atomic-scale benchmarks the modeling community had been missing. One inconsistency does remain open: millisecond laser heating in diamond anvil cells has suggested a far steeper negative melting slope, down to about 3,000 K at 50 gigapascals, in sharp conflict with both theory and the shock experiments. That tension is unresolved.
The results matter in two very different worlds. The first is fusion energy. The capsules used in ignition experiments at the National Ignition Facility are diamond shells filled with deuterium-tritium fuel, and the implosion is kicked off by three successive shocks that compress the diamond shell. Because the melting behavior was uncertain, designers set the first shock conservatively above 1.2 terapascal to stay clear of mixed solid-liquid states, a choice that limits compression. With the phase boundary now mapped, the authors argue, a gentler first shock can still drive the capsule to a uniform liquid state while keeping entropy lower and compressibility higher, potentially tripling the fusion energy output relative to today’s designs. The second world is planetary: knowing where diamond melts and survives constrains the way carbon-rich exoplanets and the ice layers of giant planets are modeled, where diamond may form and accumulate in deep interiors.
The caveats are part of the result. The experiments probe nanosecond timescales along a single-shock path, and the findings do not rule out BC8 forming through other routes, such as double shocks or compression heated close to the melting line, which is exactly what a Livermore and University of South Florida campaign is now attempting. What this study establishes is narrower and solid: at 1 terapascal, under shock, diamond stays diamond, and it melts around 7,300 kelvin, at a temperature that drops as the squeeze tightens.
Sources:
- Millot, M., Coppari, F., Lazicki, A., Kim, Y.-J., Landen, O.L., Smalyuk, V.A., Celliers, P.M. & Eggert, J.H. “Diamond melting in shock compression experiments at 1 TPa pressures.” Nature Physics, August 13, 2026. DOI: 10.1038/s41567-026-03413-1.
- Eggert, J.H. et al. “Melting temperature of diamond at ultrahigh pressure.” Nature Physics 6, 40-43 (2010). DOI: 10.1038/nphys1438.
- Lazicki, A. et al. “Metastability of diamond ramp-compressed to 2 terapascals.” Nature 589, 532-535 (2021). DOI: 10.1038/s41586-020-03140-4.
- LLNL NIF & Photon Science, “Beyond Diamond: Seeking An Elusive Phase of Carbon” (BC8 Discovery Science campaign).