Distinct vibrational signatures and complex phase behavior in metallic oxygen

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.