Phases[edit]

Phase diagram for solid oxygen

Six different phases of solid oxygen are known to exist:^[1][6]

1. α-phase: light blue – forms at 1 atm, below 23.8 K, monoclinic crystal structure.

2. β-phase: faint blue to pink – forms at 1 atm, below 43.8 K, rhombohedral crystal structure (at room temperature and high pressure begins transformation to tetraoxygen).

3. γ-phase: faint blue – forms at 1 atm, below 54.36 K, cubic crystal structure.

4. δ-phase: orange – forms at room temperature at a pressure of 9 GPa

5. ε-phase: dark-red to black – forms at room temperature at pressures greater than 10 GPa

6. ζ-phase: metallic – forms at pressures greater than 96 GPa

It has been known that oxygen is solidified into a state called the β-phase at room temperature by applying pressure, and with further increasing pressure, the β-phase undergoes phase transitions to the δ-phase at 9 GPa and the ε-phase at 10 GPa; and, due to the increase in molecular interactions, the color of the β-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 ζ-phase appears at 96 GPa when ε-phase oxygen is further compressed.^[6]

Red oxygen[edit]

Main article: Octaoxygen

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^[7] and it changes color from sky-blue to deep red.^[8] However, this is a different allotrope of oxygen, O

₈, not merely a different crystalline phase of O₂.

Ball-and-stick model of O₈

Part of the crystal structure of ε-oxygen

Metallic oxygen[edit]

A ζ-phase appears at 96 GPa when ε-phase oxygen is further compressed.^[7] This phase was discovered in 1990 by pressurizing oxygen to 132 GPa.^[3] The ζ-phase with metallic cluster^[9] exhibits superconductivity at pressures over 100 GPa and a temperature below 0.6 K.^[4][6]