Hydrocarbons under pressure: phase diagrams and surprising new compounds in the C-H system*

Understanding the high-pressure behavior of 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. Concluding our results, in C-H system only saturated hydrocarbons are thermodynamically stable. Several surprising stable methane-hydrogen co-crystals are predicted, some of them have potential energy storage purpose. We consider effects of temperature through the quasiharmonic approximation and report the pressure-temperature-composition phase diagram of the C-H system and pressure-temperature phase diagram of CH₄ (up to 2000 K).