Diamond Melts at 7,300 Kelvin: LLNL Study Closes 20-Year Gap and Points to Triple Fusion Energy Gain

Researchers at Lawrence Livermore National Laboratory (LLNL) have measured how diamond melts under extreme pressure for the first time with experimental precision, resolving a conflict that has divided theorists and experimentalists for two decades. The study, published August 13, 2026 in Nature Physics by a team led by Marius Millot, uses laser-driven shock waves to reach pressures three times greater than those at Earth's core and temperatures hotter than the surface of the Sun . The results carry direct implications for inertial confinement fusion at the National Ignition Facility and for our understanding of planetary interiors, particularly the "diamond rain" hypothesized on ice giants like Neptune and Uranus.

Four Key Findings

1. A new melting temperature of ~7,300 K closes a 20-year discrepancy

The team used the OMEGA Laser Facility at the University of Rochester to drive laser-driven shock waves through tiny diamond samples at pressures between 600 GPa and 1.8 TPa, capturing X-ray diffraction data in a billionth of a second . The measured melting point is roughly 7,300 K — over 1,000 K lower than the 2009 measurements by Jon Eggert and colleagues, and now in excellent agreement with quantum-mechanical simulations that theorists had been unable to reconcile with experiment for two decades .

2. Under a single shock, diamond skips intermediate phases and melts directly

The X-ray data showed that the diamond crystal structure persists without transforming into the predicted BC8 phase (a high-pressure carbon structure) before melting. The diamond lattice remains intact until the carbon atoms abruptly tumble into a disordered liquid . The team attributes this to kinetic trapping: under a single, rapid shock, there is not enough time for the strong carbon bonds to break and rearrange into BC8, so the system melts directly from the solid diamond structure .

3. Slower initial shocks could triple fusion energy gain at NIF

In inertial confinement fusion (ICF) experiments at the National Ignition Facility, diamond capsules hold deuterium-tritium fuel. The refined carbon model from this study shows that using slightly slower initial shockwaves — which still fully melt the diamond — would make the fusion fuel more compressible, allowing higher energy yields with the same laser drive. Simulations indicate this could increase fusion energy gain by up to threefold, provided other degradation mechanisms are controlled .

4. Diamond floats in its own melt (like ice in water)

At high pressure, solid diamond is less dense than the metallic liquid carbon it melts into, meaning solid diamond floats on liquid carbon — analogous to ice cubes floating in water . This also produces a negative Clapeyron slope over part of the melting range, where increasing pressure actually lowers the melting temperature .

Implications for Fusion Energy Research

The study directly improves ICF design models. NIF's current implosion simulations can now incorporate experimentally validated carbon equations of state, removing a key source of uncertainty in capsule performance . The predicted energy-gain increase of up to threefold is a meaningful potential boost for reaching higher-yield fusion regimes at NIF, though the researchers caution it depends on controlling other capsule degradation mechanisms . Diamond capsules are already at the center of NIF's record shots — a high-quality diamond capsule enabled NIF's 8.6 MJ fusion yield milestone on April 7, 2025 . Better understanding of diamond's phase behavior directly informs the next generation of these targets.

Implications for Planetary Science

The pressure and temperature conditions reached in the experiments are directly relevant to the interiors of ice giants like Uranus and Neptune (pressures up to ~1.8 TPa, comparable to those deep inside these planets) . The finding that diamond can coexist with and float in liquid metallic carbon helps model "diamond rain" — the long-hypothesized precipitation of diamond from liquid carbon layers within ice giant interiors. The observation that diamond persists metastably under rapid compression also informs how carbon behaves during planetary formation and dynamic events .