According to LLNL scientist Marius Millot, the team "were able to take tiny diamond samples and shock-compress them to temperatures hotter than the surface of the sun and to pressures higher than the centers of Neptune and Uranus — and still measure atomic structure, temperature, density and optical reflectivity" .

The key result: the new measurement of diamond's melting temperature agreed almost perfectly with quantum-based simulations, closing a roughly 20% discrepancy that had plagued the field for two decades . The experiments also showed that carbon remained in a diamond structure all the way until it melted, skipping an intermediate crystalline phase that some earlier experiments had suggested .

Why accuracy matters for inertial confinement fusion

At the National Ignition Facility (NIF), diamond is used as the ablator shell that encapsulates the fusion fuel during implosions . The melting behavior and equation of state that governs how the capsule responds to extreme pressure and heat are critical to achieving efficient compression.

By applying the corrected melting data, researchers believe they can "triple the energy gain" from NIF's fusion implosions through more efficient compression and better capsule design . That represents a significant step toward the goal of inertial confinement fusion as a viable energy source.

What it means for Uranus and Neptune

The experiments reached pressures and temperatures exceeding those found in the interior of ice giant planets, providing the first direct confirmation that crystalline diamond can remain stable deep inside Uranus and Neptune . The data show that diamond melts into a denser, metallic fluid under those conditions, which directly informs models of planetary structure, thermal evolution, and even the precipitation of diamond "rain" within these worlds .

This builds on earlier shock-compression studies that had already suggested diamond could be stable deep inside giant planets, showing that the melting curve has a negative Clapeyron slope at ultrahigh pressures — diamond gets denser as it melts, which is the opposite of what happens with ice at Earth's surface .

A rare resolution of theory and experiment

Discrepancies between experimental and computational results are common in high-pressure physics, but they rarely get resolved as cleanly as this. The combination of enhanced diagnostics and X-ray diffraction allowed the LLNL team to measure what earlier experiments could not, and the result matched theory within the margin of error .

The findings could also have broader significance for carbon phase behavior: diamond is the stable allotrope of carbon at pressures up to 1 terapascal, but beyond that threshold, calculations suggest the formation of a body-centered cubic structure (BC8), and at pressures above 3 TPa, a simple cubic structure is predicted . This new understanding of diamond's melting curve will refine the carbon phase diagram at the extreme conditions where these other phases may appear.

According to LLNL scientist Marius Millot, the team "were able to take tiny diamond samples and shock-compress them to temperatures hotter than the surface of the sun and to pressures higher than the centers of Neptune and Uranus — and still measure atomic structure, temperature, density and optical reflectivity" .

The key result: the new measurement of diamond's melting temperature agreed almost perfectly with quantum-based simulations, closing a roughly 20% discrepancy that had plagued the field for two decades . The experiments also showed that carbon remained in a diamond structure all the way until it melted, skipping an intermediate crystalline phase that some earlier experiments had suggested .

Why accuracy matters for inertial confinement fusion

At the National Ignition Facility (NIF), diamond is used as the ablator shell that encapsulates the fusion fuel during implosions . The melting behavior and equation of state that governs how the capsule responds to extreme pressure and heat are critical to achieving efficient compression.

By applying the corrected melting data, researchers believe they can "triple the energy gain" from NIF's fusion implosions through more efficient compression and better capsule design . That represents a significant step toward the goal of inertial confinement fusion as a viable energy source.

What it means for Uranus and Neptune

The experiments reached pressures and temperatures exceeding those found in the interior of ice giant planets, providing the first direct confirmation that crystalline diamond can remain stable deep inside Uranus and Neptune . The data show that diamond melts into a denser, metallic fluid under those conditions, which directly informs models of planetary structure, thermal evolution, and even the precipitation of diamond "rain" within these worlds .

This builds on earlier shock-compression studies that had already suggested diamond could be stable deep inside giant planets, showing that the melting curve has a negative Clapeyron slope at ultrahigh pressures — diamond gets denser as it melts, which is the opposite of what happens with ice at Earth's surface .

A rare resolution of theory and experiment

Discrepancies between experimental and computational results are common in high-pressure physics, but they rarely get resolved as cleanly as this. The combination of enhanced diagnostics and X-ray diffraction allowed the LLNL team to measure what earlier experiments could not, and the result matched theory within the margin of error .

The findings could also have broader significance for carbon phase behavior: diamond is the stable allotrope of carbon at pressures up to 1 terapascal, but beyond that threshold, calculations suggest the formation of a body-centered cubic structure (BC8), and at pressures above 3 TPa, a simple cubic structure is predicted . This new understanding of diamond's melting curve will refine the carbon phase diagram at the extreme conditions where these other phases may appear.