ε − ζ transition in solid oxygen

The structure of solid oxygen has been studied at pressures from 50 to 140 GPa using static structure search methods and molecular dynamics simulations with density functional theory and a hybrid exchange functional. Several crystalline structures with space group symmetries Pnma, $P$ ${2}₁$/m, Pm, and $P$ ${6}₃$/mmc have been identified as candidates for the $\ensuremath{\zeta}$ phase of oxygen at 0 K. Within the hybrid exchange functional framework and at 300 K temperature, $\mathit{Pm}$ is shown to be energetically most favorable above 111 GPa. A comparison with experimental x-ray diffraction, spectroscopic, and superconductivity measurements is provided for all competing structures.