Solid oxygen

This article is about the solid phase of elemental oxygen. For solids used as a source of oxygen or as an oxidizer, see Oxidizing agent.

Solid oxygen is the solid ice phase of oxygen. It forms below 54.36 K (−218.79 °C; −361.82 °F) at standard atmospheric pressure. 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 a 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.^[1] This makes solid oxygen particularly interesting, as it is considered a "spin-controlled" crystal^[1] that displays antiferromagnetic magnetic order in the low temperature phases. The magnetic properties of oxygen have been studied extensively.^[2] At very high pressures, solid oxygen changes from an insulating to a metallic state;^[3] and at very low temperatures, it transforms to a superconducting state.^[4] 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 α-phase, to 23.5 cm³/mol in the γ-phase.^[5]

Phases

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, space group C2/m (no. 12).
2. β-phase: faint blue to pink – forms at 1 atm, below 43.8 K, rhombohedral crystal structure, space group R3m (no. 166). 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, Pm3n (no. 223).^[7][8]
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 found 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

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^[9] and it changes color from sky-blue to deep red.^[10] 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

A ζ-phase appears at 96 GPa when ε-phase oxygen is further compressed.^[9] This phase was discovered in 1990 by pressurizing oxygen to 132 GPa.^[3] The ζ-phase with metallic cluster^[11] exhibits superconductivity at pressures over 100 GPa and a temperature below 0.6 K.^[4][6]