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 C2/c (4 mol/cell) and C2/m (8 mol/cell) were found as candidates for zeta-O-2. The C2/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 epsilon-zeta transition, in accordance with the suggestion based on the powder XRD measurements [Y. Akahama , 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-zeta-phase in this pressure range.