Structure of the metallic ζ-phase of oxygen and isosymmetric nature of the ε-ζ phase transition : Ab initio simulations

Using ab initio evolutionary methodology for crystal structure prediction, we explore metallic structures of oxygen at pressures in the range of 100– 250 GPa. Two energetically competitive monoclinic structures of C 2/ c 4 mol/ cell and C 2/ m 8 mol/ cell were found as candidates for -O2. The C 2/ m structure is our preferred solution, providing slightly lower enthalpy and better agreement with experimental x-ray diffraction XRD pattern and Raman frequencies. Our theoretical prediction supports the isosymmetric nature of the transition, in accordance with the suggestion based on the powder XRD measurements Y. Akahama et al., Phys. Rev. Lett. 74, 4690 1995. Moreover, we find that at least up to 250 GPa, oxygen remains a molecular solid and there is no post--phase in this pressure range.