The phase diagram of solid oxygen in the magnetic-field-temperature ($B-T$) plane is revealed by magnetization and magnetotransmission measurements. The high-field phase of solid oxygen, which we term the $\theta$ phase, is induced at the temperature below 42 K. The transition fields at the $\alpha -\theta$ and $\beta -\theta$ transitions are almost temperature independent, with the phase boundary being very steep. This phase boundary indicates that the entropy change is very small at the $\alpha -\theta$ and $\beta -\theta$ transitions, in contrast to the entropy-driven $\beta -\gamma$ transition at zero field. We argue that the $\theta$ phase differs from the high-temperature $\gamma$ phase in terms of entropy; the molecular rotation is inhibited in the $\theta$ phase, whereas permitted in the $\gamma$ phase. This finding consolidates the novelty of the $\theta$ phase.

• Received 15 May 2015

DOI:https://doi.org/10.1103/PhysRevB.92.064109