Solid oxygen forms at normal
atmospheric pressureAtmospheric pressure is defined as the force per unit area exerted against a surface by the weight of air above that surface at any given point in the Earth's atmosphere. In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above...
at a temperature below 54.36 K (−218.79 °C, −361.82 °F). Solid
oxygenOxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
O
2, like
liquid oxygenLiquid oxygen is a form of the element oxygen. It has a pale blue color and is strongly paramagnetic and can be suspended between the poles of a powerful horse shoe magnet...
, is a clear substance with a light
sky-blueDiffuse sky radiation is solar radiation reaching the Earth's surface after having been scattered from the direct solar beam by molecules or suspensoids in the atmosphere. It is also called skylight, diffuse skylight, or sky radiation and is the reason for changes in the colour of the sky...
color caused by
absorptionIn physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy, for example, to heat. The absorption of light during wave propagation...
in the red (by contrast with the blue color of the sky, which is due to
Rayleigh scatteringRayleigh scattering is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light, which may be individual atoms or molecules. It can occur when light travels in transparent solids and liquids, but is most prominently seen in gases...
of blue light).
Oxygen molecules have attracted attention because of the relationship between the molecular magnetization and
crystal structureIn mineralogy and crystallography, a crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a motif, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...
s, electronic structures, and
superconductivitySuperconductivity is a phenomenon occurring in certain materials generally at very low temperatures, characterized by exactly zero electrical resistance and the exclusion of the interior magnetic field . It was discovered by Heike Kamerlingh Onnes in 1911. Like ferromagnetism and atomic spectral...
.
Solid oxygen forms at normal
atmospheric pressureAtmospheric pressure is defined as the force per unit area exerted against a surface by the weight of air above that surface at any given point in the Earth's atmosphere. In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above...
at a temperature below 54.36 K (−218.79 °C, −361.82 °F). Solid
oxygenOxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
O
2, like
liquid oxygenLiquid oxygen is a form of the element oxygen. It has a pale blue color and is strongly paramagnetic and can be suspended between the poles of a powerful horse shoe magnet...
, is a clear substance with a light
sky-blueDiffuse sky radiation is solar radiation reaching the Earth's surface after having been scattered from the direct solar beam by molecules or suspensoids in the atmosphere. It is also called skylight, diffuse skylight, or sky radiation and is the reason for changes in the colour of the sky...
color caused by
absorptionIn physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy, for example, to heat. The absorption of light during wave propagation...
in the red (by contrast with the blue color of the sky, which is due to
Rayleigh scatteringRayleigh scattering is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light, which may be individual atoms or molecules. It can occur when light travels in transparent solids and liquids, but is most prominently seen in gases...
of blue light).
Oxygen molecules have attracted attention because of the relationship between the molecular magnetization and
crystal structureIn mineralogy and crystallography, a crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a motif, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...
s, electronic structures, and
superconductivitySuperconductivity is a phenomenon occurring in certain materials generally at very low temperatures, characterized by exactly zero electrical resistance and the exclusion of the interior magnetic field . It was discovered by Heike Kamerlingh Onnes in 1911. Like ferromagnetism and atomic spectral...
.
OxygenOxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
is the only one of the simple diatomic molecules (and one of the few molecules in general) to carry a
magnetic momentThe magnetic moment of a system is a measure of the strength and the direction of its magnetism. More technically , the term magnetic moment of a system usually refers to its magnetic dipole moment, and quantifies the contribution...
. This makes solid oxygen particularly interesting, as it is considered a 'spin-controlled'
crystalA crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is crystallography...
that displays unusual magnetic order. At very high pressures, solid oxygen changes from an
insulatingThe term thermal insulation can refer to materials used to reduce the rate of heat transfer, or the methods and processes used to reduce heat transfer....
to a
metalA metal is a chemical element that is a good conductor of both electricity and heat, forms cations and ionic bonds with non-metals. In chemistry, a metal is an element, compound, or alloy characterized by high electrical conductivity. In a metal, atoms readily lose electrons to form positive ions...
lic 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
phaseIn the physical sciences, a phase is a region of space , throughout which all physical properties of a material are essentially uniform. Examples of physical properties include density, index of refraction, and chemical composition...
s of solid oxygen are known to exist:
- α-phase: light blue - forms at 1 atm below -249.35°C, monoclinic crystal structure.
- β-phase: faint blue to pink - forms at 1 atm below -229.35°C, rhombohedral crystal structure, (at room temperature and high pressure begins transformation to tetraoxygen).
- γ-phase: faint blue - forms at 1 atm below -218.79°C, cubic crystal structure.
- δ-phase: orange — forms at room temperature by applying a pressure of 9
This is a tabulated listing of the orders of magnitude in relation to pressure expressed in pascals....
GPaThe pascal is the SI derived unit of pressure, stress, Young's modulus and tensile strength. It is a measure of force per unit area, defined as one newton per square metre...
- 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
This is a tabulated listing of the orders of magnitude in relation to pressure expressed in pascals....
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 transitionA phase transition is a natural physical process. It has the characteristic of taking a given medium with given properties and transforming some or all of that medium, into a new medium with new properties. Phase transitions occur frequently and are found everywhere in the natural world...
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 10 GPa, it undergoes a dramatic
phase transitionA phase transition is a natural physical process. It has the characteristic of taking a given medium with given properties and transforming some or all of that medium, into a new medium with new properties. Phase transitions occur frequently and are found everywhere in the natural world...
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
infraredInfrared radiation is electromagnetic radiation whose wavelength is longer than that of visible light , but shorter than that of terahertz radiation and microwaves...
absorption spectrumA material's absorption spectrum shows the fraction of incident electromagnetic radiation absorbed by the material over a range of frequencies. Atoms, for example, have absorption lines at wavelengths corresponding to the differences between the energy levels of its atomic orbitals. Each chemical...
, researchers assumed in 1999 that this phase consists of molecules in a crystal lattice. However, in 2006, it was shown by
X-ray crystallographyX-ray crystallography is a method of determining the arrangement of atoms within a crystal, in which a beam of X-rays strikes a crystal and diffracts into many specific directions. From the angles and intensities of these diffracted beams, a crystallographer can produce a three-dimensional picture...
that this stable
phaseIn the physical sciences, a phase is a region of space , throughout which all physical properties of a material are essentially uniform. Examples of physical properties include density, index of refraction, and chemical composition...
known as
ε oxygen or
red oxygen is in fact . Nobody had predicted the structure theoretically: a rhomboid cluster consisting of four molecules.
 |
 |
| Ball-and-stick model Ball-and-stick models and space-filling models are 3D or spatial molecular models which serve to display the structure of chemical products and substances or biomolecules....
of O8 |
Part of the crystal structureIn mineralogy and crystallography, a crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a motif, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...
of ε-oxygen |
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
Liquid oxygen is a form of the element oxygen. It has a pale blue color and is strongly paramagnetic and can be suspended between the poles of a powerful horse shoe magnet...
is already used as an oxidant in rocketsA rocket or rocket vehicle is a missile, spacecraft, aircraft or other vehicle which obtains thrust by the reaction of the rocket to the ejection of a jet of fast moving fluid exhaust from a rocket engine. Chemical rockets create their exhaust by the combustion of rocket propellant...
, and it has been speculated that red oxygen could make an even better oxidant, because of its higher energy densityEnergy density is a term used for the amount of energy stored in a given system or region of space per unit volume, or per unit mass, depending on the context. The latter is more formally known as specific energy...
.
- 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.