Solid oxygen

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 (by contrast with the blue color of the sky, which is due to Rayleigh scattering of blue light).

Oxygen molecules have attracted attention because of the relationship between the molecular magnetization and crystal structures, electronic structures, and superconductivity. Oxygen is the only one of the simple diatomic molecules (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 unusual magnetic order. 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.

Phase transitions
A total of 6 different phases of solid oxygen are known to exist:
 * 1) α-phase: light sky-blue —  forms at 1 atm
 * 2) β-phase: pink — forms at room temperature and high pressure
 * 3) γ-phase:  only stable below room temperature
 * 4) δ-phase: orange — forms at room temperature by applying a pressure of 9 GPa
 * 5) ε-phase: dark-red — 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 pink color of the β-phase changes into orange (δ-phase) and red (ε-phase), and the red color of the ε-phase further changes to black with increasing pressure. It was found that a ζ-phase appears at 96 GPa when ε-phase oxygen is further compressed.

Red oxygen
As the pressure of oxygen at room temperature is increased through 10 GPa, it undergoes a dramatic phase transition to a different allotrope. Its volume decreases significantly, and it changes color from blue to deep red. This ε-phase was discovered in 1979, but the structure has been unclear. Based on its infrared absorption spectrum, researchers assumed in 1999 that this phase consists of molecules in a crystal lattice. However, in 2006, it was shown by X-ray crystallography that this stable phase known as ε oxygen or red oxygen is in fact. Nobody had predicted the structure theoretically: a rhomboid cluster consisting of four  molecules.

Of all the phases of solid oxygen, this phase is particularly intriguing: it exhibits a dark-red colour, very strong infrared absorption, and a magnetic collapse. It is also stable over a very large pressure domain and has been the subject of numerous X-ray diffraction, spectroscopic and theoretical studies. It has been shown to have a monoclinic C2/m symmetry and its infrared absorption behaviour was attributed to the association of oxygen molecules into larger units.


 * Liquid oxygen is already used as a rocket fuel, and it has been speculated that red oxygen could make an even better fuel, because of its higher energy density.
 * Researchers think that this structure may greatly influence the structural investigation of elements.
 * It is the phase that forms above 600 K at pressures greater than 17 GPa.
 * At 11 GPa, the intra-cluster bond length of the cluster is 0.234 nm, and the inter-cluster distance is 0.266 nm. (For comparison, the  intra-molecular bond length of the oxygen molecule  is 0.120 nm.)
 * The formation mechanism of the cluster found in the work is not clear yet, and the researchers think that the charge transfer between oxygen molecules or the magnetic moment of oxygen molecules has a significant role in the formation.

Metallic oxygen
It was found that a ζ-phase appears at 96 GPa when ε-phase oxygen is further compressed. This phase was discovered in 1990 by pressurizing oxygen to 132 GPa. The ζ-phase with metallic luster has been known to exhibit superconductivity at low temperature. Oxygen molecules are among the very few molecules having magnetic moments, and have attracted attention because of the relationship between the molecular magnetization and crystal structures, electronic structures, and superconductivity.