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

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 CH4:H2 van der Waals inclusion compounds. We discuss the P-T phase diagram for CH (i.e. polystyrene) to illustrate that diamond formation conditions are strongly composition dependent. Finally, crystal structure searches uncover a new CH4(H2)2 van derWaals compound, the most hydrogen-rich hydrocarbon, stable between 170 and 220 GPa.