Stability and metallization of solid oxygen under high pressure

The phase diagram of oxygen is investigated for pressures from 50 to 130~GPa and temperatures up 1200 K using first principles theory. A metallic molecular structure with the \(P6_3/mmc\) symmetry (\(\eta^{'}\) phase) is determined to be thermodynamically stable in this pressure range at elevated temperatures above the \(\epsilon\)(\({O_8}\)) phase. Long-standing disagreements between theory and experiment for the stability of \(\epsilon\)(\({O_8}\)), its metallic character, and the transition pressure to the \(\zeta\) oxygen phase are resolved. Crucial for obtaining these results are the inclusion of anharmonic lattice dynamics effects and accurate calculations of exchange interactions in the presence of thermal disorder.