Dissociation of methane under high pressure

Area of Science:

• Planetary Science
• Materials Science
• Quantum Chemistry

Background:

• Methane is a key component of giant planets like Uranus and Neptune.
• Understanding methane's behavior under extreme conditions is crucial for planetary physics.
• Stable crystal forms of methane under high pressure are of fundamental interest.

Purpose of the Study:

• To predict novel insulating crystal structures of methane under extreme pressure.
• To investigate the dissociation pathways and phase diagram of methane at high pressures and temperatures.
• To reconcile experimental observations of diamond formation and methane dissociation.

Main Methods:

• Utilized the ab initio evolutionary algorithm for crystal structure prediction.
• Computed the pressure-temperature phase diagram of methane.
• Performed theoretical calculations to determine dissociation pressures and temperatures.

Main Results:

• Discovered three new insulating molecular structures of methane (P2(1)2(1)2(1), Pnma, Cmcm).
• Methane dissociates into ethane at 95 GPa, butane at 158 GPa, and carbon (diamond) + hydrogen above 287 GPa at 0 K.
• The computed phase diagram explains discrepancies in experimental diamond formation and dissociation observations.

Conclusions:

• The predicted structures provide insights into methane's behavior under extreme planetary conditions.
• The dissociation of methane into diamond and hydrogen supports theories of diamond formation in Neptune's interior.
• The study clarifies the complex phase behavior of methane relevant to planetary interiors.