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Rules of formation of H–C–N–O compounds at high pressure and the fates of planetary ices

Lewis J. Conway1,2*, Chris J. Pickard3,4*, Andreas Hermann1,2*

1 Centre for Science at Extreme Conditions, The University of Edinburgh, Edinburgh EH9 3FD, United Kingdom

2 School of Physics and Astronomy, The University of Edinburgh, Edinburgh EH9 3FD, United Kingdom

3 Department of Materials Science & Metallurgy, University of Cambridge, Cambridge CB30FS, United Kingdom

4 Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan

* Corresponding authors emails: l.j.conway@ed.ac.uk, cjp20@cam.ac.uk, a.hermann@ed.ac.uk
DOI10.24435/materialscloud:p6-zh [version v1]

Publication date: Apr 12, 2021

How to cite this record

Lewis J. Conway, Chris J. Pickard, Andreas Hermann, Rules of formation of H–C–N–O compounds at high pressure and the fates of planetary ices, Materials Cloud Archive 2021.62 (2021), doi: 10.24435/materialscloud:p6-zh.


Results of an ab initio structure search on the H+C+N+O quaternary space at 500GPa. 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 mantle–core 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, among others. Despite its importance, the H–C–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 analyze an unbiased crystal structure search among 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.

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File name Size Description
175.0 MiB 280,000+ structures generated by AIRSS and optimised by CASTEP at 500GPa. See README.
3.0 MiB Optimised geometries of low energy structures between 20 and 700GPa. See README
2.6 MiB All data used in figures in the manuscript.


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Metadata, except for email addresses, are licensed under the Creative Commons Attribution Share-Alike 4.0 International license.


H-C-N-O Chemistry High Pressure Ab Initio Random Structure Searching Planetary Ices

Version history:

2021.62 (version v1) [This version] Apr 12, 2021 DOI10.24435/materialscloud:p6-zh