The sky-blue ice that turns into a metallic superconductor

Science · Published 18 October 2026 Swipe more like this →

In short: Oxygen is usually a gas we breathe, but at extreme temperatures and pressures, it behaves in ways that defy intuition. It can crystallize into distinct colors, shift into different molecular structures, and even become a superconductor—the ultimate state of zero electrical resistance.

Solid oxygen is a unique substance in the world of chemistry. It is the only simple diatomic molecule known to carry a magnetic moment, which makes it a 'spin-controlled' crystal. This property allows scientists to study the complex relationship between molecular magnetization, crystal structures, and electronic states. Because of these interactions, solid oxygen displays antiferromagnetic magnetic order in its low-temperature phases, making it a primary subject of study for physicists interested in magnetism.

At standard atmospheric pressure, oxygen solidifies into three distinct phases as temperatures drop: the $\alpha$-phase (monoclinic) below 23.8 K, the $\beta$-phase (rhombohedral) below 43.8 K, and the $\gamma$-phase (cubic) below 54.36 K. These phases are characterized by their light sky-blue color, which is caused by the absorption of the red part of the visible light spectrum. However, when you move away from standard pressure and toward room temperature, the chemistry becomes even more dramatic.

By applying extreme pressure at room temperature, oxygen undergoes a series of phase transitions. At 9 GPa, it enters the orange $\delta$-phase; at 10 GPa, it becomes the dark-red to black $\epsilon$-phase. At pressures exceeding 10 GPa, it can even transition into a different allotrope entirely, known as octaoxygen ($\text{O}_8$). Finally, at pressures reaching 96 GPa, it transforms into a metallic $\zeta$-phase. This metallic state is capable of superconductivity at temperatures below 0.6 K when pressures exceed 100 GPa, marking a profound transition from a common gas to a high-tech quantum material.

difficulty ,3 topics [ solid-state-physics materials-science quantum-physics

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