Theory and experiments show that, with increasing pressure, the chemical bonds of methane rearrange, leading to the formation of complex polymers and then to dissociation. However, there is disagreement on the exact conditions where these changes take place. In this study, methane samples were precompressed in diamond-anvil cells and then shock compressed to pressures reaching 400 GPa, the highest pressures yet explored in methane. The results reveal a qualitative change in the Hugoniot curve at 80–150 GPa, which is interpreted as a signature of dissociation based on thermodynamic calculations and theoretical predictions.

• Received 21 April 2023
• Revised 26 October 2023
• Accepted 30 October 2023

DOI:https://doi.org/10.1103/PhysRevB.109.064102