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

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: 2CH 4 * H 2, earlier obtained experimentally, is predicted to have the I 4/ m space group and 2–90 GPa stability range at 0 K, and two new thermodynamically stable compounds 2CH 4 * 7H 2 ( P -3 m 1 space group) and CH 4 * 9H 2 ( Cm space group), and are potential energy storage materials. The P 2 1 / 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 -3 m 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 CH 4 .