Evidence for metallization in dense oxygen has been reported for over 30 years [Desgreniers et al., J. Phys. Chem. 94, 1117 (1990)] at a now routinely accessible 95 GPa [Shimizu et al., Nature 393, 767 (1998)]. However, despite the longevity of this result and the technological advances since, the nature of the metallic phase remains poorly constrained [Akahama et al., Phys. Rev. Lett. 74, 4690 (1995); Goncharov et al., Phys. Rev. B 68, 224108 (2003); Ma, Phys. Rev. B 76, 064101 (2007); and Weck et al., Phys. Rev. Lett. 102, 255503 (2009)]. In this work, through Raman spectroscopy, we report the distinct vibrational characteristics of metallic ζ-O₂ from 85 to 225 GPa. In comparison with numerical simulations, we find reasonable agreement with the C2/m candidate structure up to about 150 GPa. At higher pressures, the C2/m structure is found to be unstable and incompatible with experimental observations. Alternative candidate structures, C2/c and Ci, with only two molecules in the primitive unit cell, are found to be stable and more compatible with measurements above 175 GPa, indicative of the dissociation of (O₂)₄ units. Further, we report and discuss a strong hysteresis and metastability with the precursory phase ϵ-O₂. These findings will reinvigorate experimental and theoretical work into the dense oxygen system, which will have importance for oxygen-bearing chemistry, prevalent in the deep Earth, as well as fundamental physics.

The hydrogen molecule is made from the first and lightest element in the periodic table. When hydrogen gas is either compressed or cooled, it forms the simplest molecular solid. This solid exhibits many interesting and fundamental physical phenomena. It is believed that if the density of the solid is increased by compressing it to very high pressures, hydrogen will transform into the lightest known metal with very unusual and fascinating properties, such as room temperature superconductivity and/or superfluidity. In this article, we provide a critical look at the numerous claims of hydrogen metallization and the current experimental state of affairs.