We perform density-functional calculations of the magnetic properties of a simplified structure aimed at capturing some of the features of the elusive $\epsilon$ phase of molecular oxygen at high pressure. Starting with the $\delta$ phase—which is a quasi-two-dimensional distorted triangular arrangement of antiferromagnetically ordered molecules—pressure could decrease the $b/a$ ratio in the basal planes pushing it toward the ideal triangular value of $1/\sqrt{3},$ thus increasing magnetic frustration. We conjecture that when frustration takes over, the magnetic order may turn into a $120°$ planar spin-spiral structure inside the $\epsilon$ phase, until at higher pressures band-overlap metallization suppresses magnetization in the $\zeta$ phase. This conjecture is substantiated by calculations that also represent the attempt to apply state-of-the-art pseudopotential techniques to the magnetic properties of a frustrated antiferromagnet.

• Received 11 June 1999

DOI:https://doi.org/10.1103/PhysRevB.61.6145