Solid oxygen forms at normal atmospheric pressure at a temperature below 54.36 K (−218.79 °C, −361.82 °F). Solid oxygen O2, like liquid oxygen, is a clear substance with a light sky-blue color caused by absorption in the red part of the visible light spectrum.
Oxygen molecules have attracted attention because of the relationship between the molecular magnetization and crystal structures, electronic structures, and superconductivity. Oxygen is the only simple diatomic molecule (and one of the few molecules in general) to carry a magnetic moment. This makes solid oxygen particularly interesting, as it is considered a "spin-controlled" crystal that displays antiferromagnetic magnetic order in the low temperature phases. The magnetic properties of oxygen have been studied extensively. At very high pressures, solid oxygen changes from an insulating to a metallic state; and at very low temperatures, it even transforms to a superconducting state. Structural investigations of solid oxygen began in the 1920s and, at present, six distinct crystallographic phases are established unambiguously.
The density of solid oxygen ranges from 21 cm3/mol in the a-phase, to 23.5 cm3/mol in the g-phase.
Six different phases of solid oxygen are known to exist:
It has been known that oxygen is solidified into a state called the b-phase at room temperature by applying pressure, and with further increasing pressure, the b-phase undergoes phase transitions to the d-phase at 9 GPa and the e-phase at 10 GPa; and, due to the increase in molecular interactions, the color of the b-phase changes to pink, orange, then red (the stable octaoxygen phase), and the red color further darkens to black with increasing pressure. It was found that a metallic z-phase appears at 96 GPa when e-phase oxygen is further compressed.
As the pressure of oxygen at room temperature is increased through 10 gigapascals (1,500,000 psi), it undergoes a dramatic phase transition. Its volume decreases significantly and it changes color from sky-blue to deep red. However, this is a different allotrope of oxygen, O8, not merely a different crystalline phase of O2.
A z-phase appears at 96 GPa when e-phase oxygen is further compressed. This phase was discovered in 1990 by pressurizing oxygen to 132 GPa. The z-phase with metallic cluster exhibits superconductivity at pressures over 100 GPa and a temperature below 0.6 K.
Nascent oxygen O
Dioxygen (singlet and triplet)O2
Trioxygen (ozone and cyclic ozone) O3
Tetraoxygen O4
Octaoxygen O8
