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

• Main Authors: Lewis J. Conway, Andreas Hermann
• Format: Article
• Language: English
• Published: MDPI AG 2019-05-01
• Series: Geosciences
• Subjects: hydrocarbons; high pressure; van der Waals compounds; density functional theory; quasi-harmonic approximation
• Online Access: https://www.mdpi.com/2076-3263/9/5/227

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<inline-formula> <math display="inline"> <semantics> <msub> <mrow></mrow> <mn>4</mn> </msub> </semantics> </math> </inline-formula>:H<inline-formula> <math display="inline"> <semantics> <msub> <mrow></mrow> <mn>2</mn> </msub> </semantics> </math> </inline-formula> van der Waals inclusion compounds. We discuss the P-T phase diagram for CH and CH<inline-formula> <math display="inline"> <semantics> <msub> <mrow></mrow> <mn>2</mn> </msub> </semantics> </math> </inline-formula> (i.e., polystyrene and polyethylene) to illustrate that diamond formation conditions are strongly composition dependent. Finally, crystal structure searches uncover a new CH<inline-formula> <math display="inline"> <semantics> <msub> <mrow></mrow> <mn>4</mn> </msub> </semantics> </math> </inline-formula>(H<inline-formula> <math display="inline"> <semantics> <mrow> <msub> <mrow></mrow> <mn>2</mn> </msub> <msub> <mrow> <mo>)</mo> </mrow> <mn>2</mn> </msub> </mrow> </semantics> </math> </inline-formula> van der Waals compound, the most hydrogen-rich hydrocarbon, stable between 170 and 220 GPa.