New Phase Transition in High Pressure Molecular Oxygen

Oxygen, one of the most common and important elements in nature, has an exceedingly well explored phase diagram under pressure, up and beyond 100 GPa. We propose a subtle, yet surprisingly undetected pressure-induced phase transition of molecular O$_2$ within the $\epsilon$ phase, near 20 GPa. While thus far in the whole $\epsilon$ phase region from about 8 to 96 GPa each individual O$_2$ molecule was generally believed to be in an S=0 state, we provide theoretical results, strongly connecting with existing vibrational and structural evidence, showing that this holds only above 20 GPa, whereas there is a collective switch from S=0 to an S=1 molecular state at and below 20 GPa. The former $\epsilon$ phase thus breaks up into two: a spinless $\epsilon_0$ (20-96 GPa), and $\epsilon_1$ (8-20 GPa) where the molecules possess spin with at least short range antiferromagnetic correlations.