In the depths of ice giant planets such as Neptune and Uranus, and in inertial confinement fusion targets, chemical reactions can be significantly affected by the extreme pressure and temperature conditions. One of the most intriguing phenomena is the possible formation of diamond from hydrocarbon fluids. Despite experimental observations of diamond formation, the underlying mechanism remains incompletely understood from a first-principles, density functional theory (DFT) perspective. In this work, we provide evidence for the survival and structural evolution of a preexisting diamond phase using large-scale DFT-based and machine-learned molecular-dynamics simulations. Our reverse strategy model, initialized with a diamond structure surrounded by hydrogen, demonstrates that diamond can persist in contact with hydrogen-rich fluid at extreme pressures and temperatures relevant to ice-giant mantles. The time evolution of these systems reveals partial diamond dissolution followed by recovery of the diamond fraction through C–H bond breaking and C–C bond formation. Our Kubo-Greenwood formalism-based transport calculations reveal that the diamond-hydrogen system has lower electrical and thermal conductivity than the random hydrocarbon mixture at 150 GPa and 3000–4000 K. We expect these results to have important implications in the modeling of the structure and magnetic fields of the ice giant planet interiors and improve future inertial fusion energy target designs.

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• High-pressure studies
• Nucleation
• Phase separation
• Diamond
• Ab initio molecular dynamics