Persistence and Eventual Demise of Oxygen Molecules at Terapascal Pressures

Computational searches for structures of solid oxygen under high pressures in the multi-TPa range are carried out using density-functional-theory methods. We find that molecular oxygen persists to about 1.9 TPa at which it transforms into a semiconducting square-spiral-like polymeric structure ($I{4}_1/acd$) with a band gap of $\sim 3.0\mathrm{eV}$. Solid oxygen forms a metallic zigzag chainlike structure ($Cmcm$) at about 3.0 TPa, but the chains in each layer gradually merge as the pressure is increased and a structure of $Fmmm$ symmetry forms at about 9.3 TPa in which each atom has four nearest neighbors. The superconducting properties of molecular oxygen do not vary much with compression, although the structure becomes more symmetric. The electronic properties of oxygen have a complex evolution with pressure, swapping between insulating, semiconducting, and metallic.

Figure 1

Enthalpy-pressure relations for solid oxygen. The upper right inset shows the enthalpies of the molecular phases, while the insets in the lower left corner show the square-spiral structure of $I{4}_1/acd$ and zigzag chains of $Cmcm$, respectively. The green bubbles represent the electron lone pairs.

Figure 2

Electronic band structures and projected densities of states (PDoS). (a) $R\overline{3}m$ at 1.8 TPa, (b) $I{4}_1/acd$ at 2.0 TPa, and (c) $Cmcm$ at 3.5 TPa. The zero of energy is at the Fermi level.

Figure 3

Electronic densities of polymeric oxygen. (a) and (b) $I{4}_1/acd$ at 2.0 TPa along the [100] and [001] directions, (c) and (d) $Cmcm$ at 3.5 TPa along the [100] and [001] directions.

Figure 4

Phonon dispersion relations of $R\overline{3}m$ at 1.8 TPa (upper), $I{4}_1/acd$ at 2.0 TPa (lower left) and $Cmcm$ at 3.5 TPa (lower right). The blue circles with magenta filling show the phonon linewidth at different wave vectors.