The stakes of this question extend far beyond materials curiosity. Carbon’s phase diagram at extreme pressures underpins our understanding of the interiors of giant planets such as Neptune and Uranus, where carbon-rich matter is thought to exist in exotic superionic or molten states. It also matters for inertial confinement fusion, where laser-driven capsules containing carbon-based materials are compressed to terapascal conditions in the quest for controlled nuclear fusion. Accurate knowledge of where diamond melts—and at what temperature—directly informs the design and interpretation of those experiments, and may even help researchers achieve higher energy gain in laser-driven fusion schemes.

For decades, scientists have disagreed about carbon’s behavior at these crushing pressures. Density functional theory, the workhorse computational method of condensed matter physics, predicts that above roughly 1 TPa, carbon should abandon the tetrahedrally bonded diamond structure in favor of a denser arrangement known as the BC8 phase—a crystal geometry previously observed only in silicon and germanium under compression. Yet experimental attempts to detect this transition had produced conflicting results, and earlier reports had claimed evidence for a transition to BC8 at somewhat lower pressures. The new study contradicts that picture directly. By combining several independent diagnostic techniques, the team found no sign of a phase transition; instead, the diamond lattice survives all the way to the terapascal regime.

The experiments were carried out on microcrystalline diamond samples subjected to nanosecond-duration shock compression, a technique in which an intense laser or gas gun drives a compression wave through the sample faster than sound can propagate. Shock compression is uniquely suited to reaching terapascal pressures because it concentrates enormous energy into a brief moment, but the price is that any measurement must be made within that fleeting window before the sample disintegrates. The researchers rose to this challenge by deploying a trio of complementary diagnostics on the same shots: optical velocimetry to track the motion of the shock front, pyrometry to measure the light emitted by the heated sample and infer its temperature, and X-ray diffraction to interrogate the atomic arrangement directly.

The combination proved decisive. Optical velocimetry provided the pressure and density state of the compressed diamond, while pyrometry revealed the shock temperature, climbing to values near 7,300 kelvin at the highest pressures. X-ray diffraction, meanwhile, delivered the atomic-scale verdict. As the shock pressure rose, the intensity of the diffraction signal from the diamond lattice decreased steadily—a hallmark of melting, in which the long-range order of the crystal breaks down and the coherent scattering of X-rays fades. Crucially, the diffraction pattern retained the signature of the diamond structure at all pressures reached, showing no evidence of the BC8 phase that theory had predicted should intervene.

The temperature and reflectivity measurements added another layer of insight. Changes in the thermodynamic and optical properties of the compressed material, recorded alongside the weakening diffraction, allowed the team to construct a melting curve for diamond in this regime. The result came with a surprise: rather than rising steeply with pressure as many models had assumed, the melting temperature shows a slight decrease as pressure increases near 7,300 kelvin. This shallow negative slope, similar in spirit to the melting behavior of ice under pressure, provides a sensitive benchmark against which theoretical models can now be tested.

That benchmark is precisely what the field has been waiting for. Simulations of matter under extreme conditions rely on quantum mechanical treatments of the electrons, and different approximations can yield melting curves differing by thousands of kelvin. The new experimental data anchor those calculations to reality, delivering atomic-scale benchmarks for quantum simulations of condensed matter at extreme conditions. In doing so, the study resolves the discrepancy between experiments and simulations that has shadowed carbon’s high-pressure melting line for years, and establishes that any transition to BC8, if it occurs at all, must lie at pressures beyond 1 TPa.

The implications ripple outward to planetary science. Inside icy giant planets, carbon—delivered by methane and other hydrocarbons—experiences pressures and temperatures squarely in the range probed by these experiments. Whether carbon there exists as solid diamond, liquid, or something stranger depends on the melting curve now measured. A melting line that declines with pressure means that as material sinks deeper into a planet’s interior, conditions may favor melting over solidification in ways that existing models had not captured, potentially reshaping predictions about planetary heat flows, magnetic field generation, and internal layering. Diamond rain scenarios, in which crystallized carbon falls through planetary mantles, must now be evaluated against a revised thermal landscape.

The fusion connection is equally consequential. In laser-driven inertial confinement fusion, ablator materials and fuel containers experience shock pressures in the terapascal range during the implosion that ignites fusion reactions. Understanding how diamond behaves—how it compresses, heats, and melts—helps scientists model the implosion hydrodynamics with greater fidelity, and the authors note that the improved understanding of diamond melting might contribute to achieving higher energy gain. Every refinement in the equation of state of carbon translates into tighter control over the conditions needed to coax net energy from a fusion target.

The achievement also represents a triumph of experimental technique. Simultaneously recording velocimetry, pyrometry, and X-ray diffraction during a nanosecond shock event requires exquisite timing, intense X-ray sources, and detectors capable of capturing a single diffraction snapshot before the sample is destroyed. The success of the multi-diagnostic approach sets a template for future studies of other light elements—oxygen, nitrogen, hydrogen—whose high-pressure behavior governs the interiors of a wide range of planets and the performance of fusion targets. Each element probed with this level of rigor adds another anchor for the theory of dense matter.

With the diamond melting curve now measured at 1 TPa and the BC8 transition pushed beyond experimental reach, carbon’s phase diagram at the highest pressures comes into sharper focus than ever before. The work demonstrates that even the most familiar substance on Earth can still conceal fundamental surprises when driven to states found only in planetary interiors and stellar furnaces—and that with the right combination of lasers, X-rays, and ingenuity, those states can be witnessed in the laboratory for the few billionths of a second they exist.