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High Pressure Hydrocarbons Revisited: From van der Waals Compounds to Diamond

1
Centre for Science at Extreme Conditions, School of Physics and Astronomy, The University of Edinburgh, Edinburgh EH9 3FD, UK
2
Scottish Universities Physics Alliance, School of Physics and Astronomy, The University of Edinburgh, Edinburgh EH9 3FD, UK
*
Author to whom correspondence should be addressed.
Geosciences 2019, 9(5), 227; https://doi.org/10.3390/geosciences9050227
Received: 18 March 2019 / Revised: 26 April 2019 / Accepted: 8 May 2019 / Published: 16 May 2019
(This article belongs to the Special Issue Interiors of Icy Ocean Worlds)
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Abstract

Methane and other hydrocarbons are major components of the mantle regions of icy planets. Several recent computational studies have investigated the high-pressure behaviour of specific hydrocarbons. To develop a global picture of hydrocarbon stability, to identify relevant decomposition reactions, and probe eventual formation of diamond, a complete study of all hydrocarbons is needed. Using density functional theory calculations we survey here all known C-H crystal structures augmented by targeted crystal structure searches to build hydrocarbon phase diagrams in the ground state and at elevated temperatures. We find that an updated pressure-temperature phase diagram for methane is dominated at intermediate pressures by CH 4 :H 2 van der Waals inclusion compounds. We discuss the P-T phase diagram for CH and CH 2 (i.e., polystyrene and polyethylene) to illustrate that diamond formation conditions are strongly composition dependent. Finally, crystal structure searches uncover a new CH 4 (H 2 ) 2 van der Waals compound, the most hydrogen-rich hydrocarbon, stable between 170 and 220 GPa. View Full-Text
Keywords: hydrocarbons; high pressure; van der Waals compounds; density functional theory; quasi-harmonic approximation hydrocarbons; high pressure; van der Waals compounds; density functional theory; quasi-harmonic approximation
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited (CC BY 4.0).
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Conway, L.J.; Hermann, A. High Pressure Hydrocarbons Revisited: From van der Waals Compounds to Diamond. Geosciences 2019, 9, 227.

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