Hydrocarbons under Pressure: Phase Diagrams and Surprising New Compounds in the C–H System

• Published in: Journal of physical chemistry. C Vol. 123; no. 33; pp. 20497 - 20501
• Main Authors: Naumova, Anastasia S, Lepeshkin, Sergey V, Oganov, Artem R
• Format: Journal Article
• Language: English
• Published: American Chemical Society 22.08.2019
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.9b01353

Understanding the high-pressure behavior of the C–H system is of great importance due to its key role in organic, bio-, petroleum, and planetary chemistry. We have performed a systematic investigation of the pressure–composition phase diagram of the C–H system at pressures up to 400 GPa using evolutionary structure prediction coupled with ab initio calculations and discovered that only saturated hydrocarbons are thermodynamically stable. Several stable methane-hydrogen cocrystals are predicted: 2CH4 * H2, earlier obtained experimentally, is predicted to have the I4/m space group and 2–90 GPa stability range at 0 K, and two new thermodynamically stable compounds 2CH4 * 7H2 (P-3m1 space group) and CH4 * 9H2 (Cm space group), and are potential energy storage materials. The P21/c phase of methane is predicted to be stable at pressures <8 GPa; bulk graphane (CH) was shown to be thermodynamically stable at 7–18 and 18–50 GPa and 0 K in the P-3m and Cmca phases, respectively; polyethylene is shown to have a narrow field of stability. We report the p–T–x phase diagram of the C–H system and p–T phase diagram of CH4.