Solid oxygen is the solid ice phase of oxygen. It forms below at standard

atmospheric pressure Atmospheric pressure, also known as air pressure or barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1,013. ...

. Solid

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

O₂, like

liquid oxygen Liquid oxygen, sometimes abbreviated as LOX or LOXygen, is a clear cyan liquid form of dioxygen . It was used as the oxidizer in the first liquid-fueled rocket invented in 1926 by Robert H. Goddard, an application which is ongoing. Physical ...

, 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 structure In crystallography, crystal structure is a description of ordered arrangement of atoms, ions, or molecules in a crystalline material. Ordered structures occur from intrinsic nature of constituent particles to form symmetric patterns that repeat ...

electronic structure Quantum chemistry, also called molecular quantum mechanics, is a branch of physical chemistry focused on the application of quantum mechanics to chemical systems, particularly towards the quantum-mechanical calculation of electronic contributions ...

s, and

superconductivity Superconductivity is a set of physical properties observed in superconductors: materials where Electrical resistance and conductance, electrical resistance vanishes and Magnetic field, magnetic fields are expelled from the material. Unlike an ord ...

. Oxygen is the only simple

diatomic molecule Diatomic molecules () are molecules composed of only two atoms, of the same or different chemical elements. If a diatomic molecule consists of two atoms of the same element, such as hydrogen () or oxygen (), then it is said to be homonuclear mol ...

(and one of the few molecules in general) to carry a

magnetic moment In electromagnetism, the magnetic moment or magnetic dipole moment is the combination of strength and orientation of a magnet or other object or system that exerts a magnetic field. The magnetic dipole moment of an object determines the magnitude ...

. This makes solid oxygen particularly interesting, as it is considered a "spin-controlled"

crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...

that displays

antiferromagnetic In materials that exhibit antiferromagnetism, the magnetic moments of atoms or molecules, usually related to the spins of electrons, align in a regular pattern with neighboring Spin (physics), spins (on different sublattices) pointing in oppos ...

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

metal A metal () is a material that, when polished or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, electricity and thermal conductivity, heat relatively well. These properties are all associated wit ...

lic state; and at very low temperatures, it 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 α-phase, to 23.5 cm³/mol in the γ-phase.

Phases

Six different phases of solid oxygen are known to exist: # α-phase: ''light blue'' forms at 1 atm, below 23.8 K,

monoclinic In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three Vector (geometric), vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in t ...

crystal structure,

space group In mathematics, physics and chemistry, a space group is the symmetry group of a repeating pattern in space, usually in three dimensions. The elements of a space group (its symmetry operations) are the rigid transformations of the pattern that ...

''C''2/''m'' (no. 12). # β-phase: ''faint blue'' to ''pink'' forms at 1 atm, below 43.8 K, rhombohedral crystal structure, space group ''R'm'' (no. 166). At room temperature and high pressure begins transformation to tetraoxygen. # γ-phase: ''faint blue'' forms at 1 atm, below 54.36 K, cubic crystal structure, ''Pm'n'' (no. 223). # δ-phase: ''orange'' forms at room temperature at a pressure of 9

GPa Grading in education is the application of standardized measurements to evaluate different levels of student achievement in a course. Grades can be expressed as letters (usually A to F), as a range (for example, 1 to 6), percentages, or as num ...

# ε-phase: ''dark-red'' to ''black'' forms at room temperature at pressures greater than 10 GPa # 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 transition In physics, chemistry, and other related fields like biology, a phase transition (or phase change) is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic Sta ...

s 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.

Red oxygen

As the pressure of oxygen at room temperature is increased through , it undergoes a dramatic

. Its volume decreases significantly and it changes color from sky-blue to deep red. However, this is a different allotrope of oxygen, , not merely a different crystalline phase of O₂.

Metallic oxygen

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 cluster exhibits superconductivity at pressures over 100 GPa and a temperature below 0.6 K.

References

{{oxygenallotropes Crystals in space group 12 Crystals in space group 166 Crystals in space group 221 Oxygen Cryogenics Ice