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

TL;DR

This study systematically explores the high-pressure phase diagram of the C-H system up to 400 GPa, predicting new stable compounds and phases relevant to planetary and energy storage applications.

Contribution

It introduces new thermodynamically stable hydrocarbons and co-crystals under high pressure, expanding understanding of C-H system behavior.

Findings

Predicted stable methane-hydrogen co-crystals, including 2CH4 * H2, 2CH4 * 7H2, and CH4 * 9H2.

Identified stability ranges for various phases like P21/c methane and bulk graphane.

Mapped the comprehensive p-T-x phase diagram of the C-H system.

Abstract

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. Several stable methane-hydrogen co crystals are predicted: 2CH4 * H2, earlier obtained experimentally, is predicted to have 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), as potential energy storage materials. 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.