X-ray diffraction study of single crystal methane in helium up to above 300 GPa: metallization or dissociation into diamond?

At 300K, CH4 was shown to be stable up to 200 GPa and structural changes have been reported from Raman and XRD. On the other hand, calculations predict that CH4 should become unstable under pressure and a transformation into C2H6 above 60 GPa and then diamond above 300 GPa should be observed. Is CH4 reluctant to dissociation because of kinetic hindrance or because calculations were not considering the stable structure? A single crystal diffraction reported that the structure of phase B of CH4 at about 8 GPa is indeed more complex than expected. In our previous proposal HC-4449, we were able to confirm this structure and showed that it remained stable up to 120 GPa at least. The aim of the present study is to extend the XRD structural measurements of CH4 up to above 300 GPa. There are two possible important outcomes: 1) CH4 remains stable up to extreme pressures and its metallization could be observed with possible superconductivity; 2) CH4 transforms into diamond and hydrogen.