Ab initio calculation of the miscibility diagram for mixtures of hydrogen and water

We calculate the miscibility gap in mixtures of hydrogen and water under high-temperature and high-pressure conditions as relevant for planetary interiors with density functional theory combined with classical molecular dynamics. In contrast to earlier calculations, we find a miscibility gap at temperatures below 1500--2000 K at pressures of up to 300 kbar, which extends the experimentally known immiscibility region by one order of magnitude in pressure. In contrast to extrapolated experimental demixing lines reaching to high temperatures, our results indicate a termination of the demixing region close to 2000 K. This finding profoundly impacts the understanding of the interiors of ice-giant planets such as Neptune and Uranus by supporting a partially demixed interior, including a density discontinuity near 2000 K, which corresponds to planetary radii of 0.85--0.95 in Uranus and Neptune. Additionally, our findings are relevant for thermal evolution models of Earth that aim to explain the formation of superreducing mineral associations.