Stable H-C-N-O compounds at high pressure: rules of formation and the fates of planetary ices

Lewis J Conway, Chris J. Pickard, Andreas Hermann

Research output: Contribution to journalArticlepeer-review

Abstract

The solar system’s outer planets, and many of their moons, are dominated
by matter from the H-C-N-O chemical space, based on solar system abundances of hydrogen and the planetary ices H2O, CH4, and NH3. In the planetary interiors, these ices will experience extreme pressure conditions, around 5 Mbar at the Neptune mantlecore boundary, and it is expected that they undergo phase transitions, decompose, and form entirely new compounds. While temperature
will dictate the formation of compounds, ground-state density functional theory allows us to probe the chemical effects resulting from pressure alone. These structural developments in turn determine the planets’ interior structures, thermal evolution, and magnetic field generation, amongst others. Despite its importance, the HC-N-O system has not been surveyed systematically to explore which
compounds emerge at high-pressure conditions, and what governs their stability. Here, we report on and analyse an unbiased crystal structure search amongst H-C-N-O compounds between 1 and 5 Mbar. We demonstrate that simple chemical rules drive stability in this composition space, which explains why the simplest possible quaternary mixture HCNO – isoelectronic to diamond – emerges as a stable compound, and discuss dominant decomposition products of planetary ice mixtures.
Original languageEnglish
JournalProceedings of the National Academy of Sciences
Publication statusAccepted/In press - 25 Mar 2021

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