Researchers using the OMEGA Laser Facility shock-compressed diamond to pressures near one terapascal and temperatures hotter than the Sun, discovering that the material melts directly from a cubic crystal lattice into liquid carbon without forming intermediate structures, while resolving a 20-year temperature mismatch.
Diamond possesses a formidable reputation as the hardest natural material on Earth, enduring conditions that would vaporize most other substances. Yet when subjected to extreme forces, even this unyielding lattice yields. Recent high-energy physics experiments reveal that diamond melts directly from its familiar cubic crystal structure into liquid carbon without transitioning through predicted intermediate phases.
Lasers at the Omega Facility Push Carbon Past Terapascal Thresholds
To recreate the crushing environments found deep within ice giant planets, physicists turned to high-powered optics. Operating at the OMEGA Laser Facility in Rochester, N.Y., researchers directed intense laser pulses onto tiny synthetic diamond targets. These pulses launched rapid shock waves through the samples, driving pressures upward from roughly 600 gigapascals to a staggering 1.8 terapascals—nearly 10 million times atmospheric pressure at sea level.
Capturing data under these fleeting, violent conditions requires split-second instrumentation. The experimental setups survived for only nanoseconds, during which diagnostic tools tracked shock speed, optical reflectivity, and material brightness. Concurrently, advanced X-ray diffraction recorded how the atomic arrangement shifted as the immense pressures mounted.
Solving a Decades-Old Mismatch in Melting Temperature
The campaign successfully settled a roughly 20-year scientific disagreement regarding diamond’s precise melting point under ultrahigh pressures. A landmark 2009 study demonstrated that diamond melts into an electrically conducting fluid that is denser than the solid phase. However, temperature estimates from that earlier work persistently conflicted with modern quantum-mechanical computer simulations.
According to the researchers, the gap between theoretical models and laboratory observations spanned approximately 1,500 kelvin. Theorists refined calculations repeatedly, yet the discrepancy stubbornly remained. The newer diagnostics revealed that original temperature measurements had been skewed by more than 1,000 degrees Celsius.
With refined diagnostic techniques, the team placed diamond’s melting temperature at approximately 7,300 kelvin—roughly 7,000 degrees Celsius or 12,700 Fahrenheit—at a pressure of one terapascal. This revised figure brings experimental observations into close harmony with theoretical predictions, validating advanced simulation techniques that could not previously reproduce the laboratory outcomes.
The Puzzle of the Missing BC8 Phase and Metallic Fluid Density
Beyond resolving the temperature debate, the experiments addressed a second major mystery involving a hypothetical high-pressure carbon structure known as BC8. Theoretical calculations had predicted that the regular cubic diamond lattice should temporarily transform into BC8 before fully melting at pressures near one terapascal. Earlier shock experiments were interpreted as evidence that this transition initiated above 900 gigapascals.
The X-ray diffraction data, however, told a different story. As pressure increased, the diffraction signal from crystalline diamond gradually weakened and eventually vanished, but no distinct BC8 phase appeared. Crystalline diamond and liquid carbon simply coexisted until the lattice dissolved entirely.
Because the single-shock compression lasts mere nanoseconds, the carbon atoms remain trapped
in their original diamond structure until thermal energy forces complete disorder. In this narrow pressure regime, solid diamond can actually float atop molten carbon, mirroring the familiar behavior of ice cubes in water.
Implications for Planetary Interiors and Inertial Confinement Fusion
These findings extend far beyond fundamental high-pressure physics, offering direct insight into extreme astrophysical environments. Understanding how carbon behaves at high pressures and temperatures refines scientific models of the deep interiors of ice giant planets such as Neptune and Uranus.
Furthermore, the data holds practical value for energy research. Diamond is frequently utilized to fabricate the ablator capsules that surround fusion fuel in laser-driven inertial confinement fusion experiments. Accurate equations of state for carbon under extreme compression are essential for improving capsule design and optimizing target gains in fusion facilities.
