Solid oxygen ζ phase and its transition from ε phase at extremely high pressure: A first-principles analysis

The epsilon and zeta phases of solid oxygen are studied with periodic hybrid Kohn-Sham calculations and atomic basis sets. The evolution of the lattice parameters and transition pressure are in excellent agreement with experiment. The greater stability of the zeta phase at high pressure is mainly due to thermal contributions. Phonon calculations indicate that the experimental C2/m (O-2)4 unit cell for the zeta phase actually arises as a time average of P1 reduced-symmetry (O-2)16 unit cells. The structural distortion of the reduced-symmetry zeta structure gives rise to a mechanism that could explain the superconductivity of the zeta phase.