Allotropes of carbon

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Under certain conditions, carbon can be found in its atomic form. It can be formed by vaporizing graphite, by passing large electric currents to form a

Diatomic carbon can also be found under certain conditions. It is often detected via spectroscopy in extraterrestrial bodies, including comets and certain stars.^[3][4]

Diamond

Diamond is a well-known allotrope of carbon. The hardness, extremely high refractive index, and high dispersion of light make diamond useful for industrial applications and for jewelry. Diamond is the hardest known natural mineral. This makes it an excellent abrasive and makes it hold polish and luster extremely well. No known naturally occurring substance can cut or scratch diamond, except another diamond. In diamond form, carbon is one of the costliest elements.

The crystal structure of diamond is a

The market for industrial-grade diamonds operates much differently from its gem-grade counterpart. Industrial diamonds are valued mostly for their hardness and heat conductivity, making many of the gemological characteristics of diamond, including clarity and color, mostly irrelevant. This helps explain why 80% of mined diamonds (equal to about 100 million carats or 20 tonnes annually) are unsuitable for use as gemstones and known as bort, are destined for industrial use. In addition to mined diamonds, synthetic diamonds found industrial applications almost immediately after their invention in the 1950s; another 400 million carats (80 tonnes) of synthetic diamonds are produced annually for industrial use, which is nearly four times the mass of natural diamonds mined over the same period.

With the continuing advances being made in the production of synthetic diamond, future applications are beginning to become feasible. Garnering much excitement is the possible use of diamond as a semiconductor suitable to build microchips from, or the use of diamond as a heat sink in electronics. Significant research efforts in Japan, Europe, and the United States are under way to capitalize on the potential offered by diamond's unique material properties, combined with increased quality and quantity of supply starting to become available from synthetic diamond manufacturers.^{[citation needed]}

Graphite

Graphite, named by

Graphite is the most stable allotrope of carbon. Contrary to popular belief, high-purity graphite does not readily burn, even at elevated temperatures.[8] For this reason, it is used in nuclear reactors and for high-temperature crucibles for melting metals.^[9] At very high temperatures and pressures (roughly 2000 °C and 5 GPa), it can be transformed into diamond.^{[citation needed]}

Natural and crystalline graphites are not often used in pure form as structural materials due to their shear-planes, brittleness and inconsistent mechanical properties.

In its pure glassy (isotropic) synthetic forms,

Graphite is slightly more reactive than diamond. This is because the reactants are able to penetrate between the hexagonal layers of carbon atoms in graphite. It is unaffected by ordinary solvents, dilute acids, or fused alkalis. However, chromic acid oxidizes it to carbon dioxide.

Graphene

A single layer of graphite is called graphene and has extraordinary electrical, thermal, and physical properties. It can be produced by epitaxy on an insulating or conducting substrate or by mechanical exfoliation (repeated peeling) from graphite. Its applications may include replacing silicon in high-performance electronic devices. With two layers stacked, bilayer graphene results with different properties.

Lonsdaleite (hexagonal diamond)

Graphenylene[13] is a single layer carbon material with biphenylene-like subunits as basis in its hexagonal lattice structure. It is also known as biphenylene-carbon.

Carbophene

Carbophene is a 2 dimensional covalent organic framework.^[14] 4-6 carbophene has been synthesized from 1-3-5 trihydroxybenzene. It consists of 4-carbon and 6-carbon rings in 1:1 ratio. The angles between the three σ-bonds of the orbitals are approximately 120°, 90°, and 150°.^[15]

AA'-graphite

AA'-graphite is an allotrope of carbon similar to graphite, but where the layers are positioned differently to each other as compared to the order in graphite.

Diamane

Diamane is a 2D form of diamond. It can be made via high pressures, but without that pressure, the material reverts to graphene. Another technique is to add hydrogen atoms, but those bonds are weak. Using fluorine (xenon-difluoride) instead brings the layers closer together, strengthening the bonds. This is called f-diamane.^[16]

Amorphous carbon

Amorphous carbon is the name used for carbon that does not have any crystalline structure. As with all glassy materials, some short-range order can be observed, but there is no long-range pattern of atomic positions. While entirely amorphous carbon can be produced, most amorphous carbon contains microscopic crystals of graphite-like,^[17] or even diamond-like carbon.^[18]

Coal and soot or carbon black are informally called amorphous carbon. However, they are products of pyrolysis (the process of decomposing a substance by the action of heat), which does not produce true amorphous carbon under normal conditions.

Nanocarbons

Buckminsterfullerenes

The buckminsterfullerenes, or usually just fullerenes or buckyballs for short, were discovered in 1985 by a team of scientists from Rice University and the University of Sussex, three of whom were awarded the 1996 Nobel Prize in Chemistry. They are named for the resemblance to the geodesic structures devised by Richard Buckminster "Bucky" Fuller. Fullerenes are positively curved molecules of varying sizes composed entirely of carbon, which take the form of a hollow sphere, ellipsoid, or tube (the C60 version has the same form as a traditional stitched soccer ball).

As of the early twenty-first century, the chemical and physical properties of fullerenes are still under heavy study, in both pure and applied research labs. In April 2003, fullerenes were under study for potential medicinal use — binding specific antibiotics to the structure to target resistant bacteria and even target certain cancer cells such as melanoma.

Carbon nanotubes

Carbon nanotubes, also called buckytubes, are cylindrical

Carbon nanobuds are a newly discovered allotrope of

Schwarzites

Schwarzites are negatively curved carbon surfaces originally proposed by decorating triply periodic minimal surfaces with carbon atoms. The geometric topology of the structure is determined by the presence of ring defects, such as heptagons and octagons, to graphene's hexagonal lattice.^[19] (Negative curvature bends surfaces outwards like a saddle rather than bending inwards like a sphere.)

Recent work has proposed zeolite-templated carbons (ZTCs) may be schwarzites. The name, ZTC, derives from their origin inside the pores of zeolites, crystalline silicon dioxide minerals. A vapor of carbon-containing molecules is injected into the zeolite, where the carbon gathers on the pores' walls, creating the negative curve. Dissolving the zeolite leaves the carbon. A team generated structures by decorating the pores of a zeolite with carbon through a Monte Carlo method. Some of the resulting models resemble schwarzite-like structures.^[20]

Glassy carbon

Glassy carbon or vitreous carbon is a class of non-graphitizing carbon widely used as an electrode material in electrochemistry, as well as for high-temperature crucibles and as a component of some prosthetic devices.

It was first produced by Bernard Redfern in the mid-1950s at the laboratories of The Carborundum Company, Manchester, UK. He had set out to develop a polymer matrix to mirror a diamond structure and discovered a resole (phenolic) resin that would, with special preparation, set without a catalyst. Using this resin, the first glassy carbon was produced.

The preparation of glassy carbon involves subjecting the organic precursors to a series of heat treatments at temperatures up to 3000 °C. Unlike many non-graphitizing carbons, they are impermeable to gases and are chemically extremely inert, especially those prepared at very high temperatures. It has been demonstrated that the rates of oxidation of certain glassy carbons in oxygen, carbon dioxide or water vapor are lower than those of any other carbon. They are also highly resistant to attack by acids. Thus, while normal graphite is reduced to a powder by a mixture of concentrated sulfuric and nitric acids at room temperature, glassy carbon is unaffected by such treatment, even after several months.

Carbon nanofoam

Carbon nanofoam is the fifth known allotrope of carbon, discovered in 1997 by Andrei V. Rode and co-workers at the Australian National University in Canberra. It consists of a low-density cluster-assembly of carbon atoms strung together in a loose three-dimensional web.

Each cluster is about 6 nanometers wide and consists of about 4000 carbon atoms linked in graphite-like sheets that are given negative curvature by the inclusion of heptagons among the regular hexagonal pattern. This is the opposite of what happens in the case of buckminsterfullerenes, in which carbon sheets are given positive curvature by the inclusion of pentagons.

The large-scale structure of carbon nanofoam is similar to that of an

Carbide-derived carbon

Carbide-derived carbon (CDC) is a family of carbon materials with different surface geometries and carbon ordering that are produced via selective removal of metals from metal carbide precursors, such as TiC, SiC, Ti₃AlC₂, Mo₂C, etc. This synthesis is accomplished using chlorine treatment, hydrothermal synthesis, or high-temperature selective metal desorption under vacuum. Depending on the synthesis method, carbide precursor, and reaction parameters, multiple carbon allotropes can be achieved, including endohedral particles composed of predominantly amorphous carbon, carbon nanotubes, epitaxial graphene, nanocrystalline diamond, onion-like carbon, and graphitic ribbons, barrels, and horns. These structures exhibit high porosity and specific surface areas, with highly tunable pore diameters, making them promising materials for supercapacitor-based energy storage, water filtration and capacitive desalinization, catalyst support, and cytokine removal.^[21]

Other metastable carbon phases, some diamondlike, have been produced from reactions of SiC or CH3SiCl3 with CF4.^[22]

Linear acetylenic carbon

A one-dimensional carbon polymer with the structure —(C≡C)_n—. Its structure is relatively like that of Amorphous carbon.

Cyclocarbons

Cyclo[18]carbon (C₁₈) was synthesized in 2019.^[23]

Other possible allotropes

Many other allotropes have been hypothesized but have yet to be synthesized.

• body-centered cubic structure. This phase has importance in astrophysics and deep interiors of planets like Uranus and Neptune. Various structures have been proposed. Superdense and superhard material resembling this phase was synthesized and published in 1979 and reported to have the Im3 space group with eight atoms per primitive unit cell (16 atoms per conventional unit cell).^[24] Claims were made that the so-called C₈ structure had been synthesized, having eight-carbon cubes similar to cubane in the Im3m space group, with eight atoms per primitive unit cell, or 16 atoms per conventional unit cell (also called supercubane, see illustration to the right). But a paper in 1988 claimed that a better theory was that the structure was the same as that of an allotrope of silicon called Si-III or γ-silicon, the so-called BC8 structure with space group Ia3 and 8 atoms per primitive unit cell (16 atoms per conventional unit cell).^[25][26] In 2008 it was reported that the cubane-like structure had been identified.^[27][28] A paper in 2012 considered four proposed structures, the supercubane structure, the BC8 structure, a structure with clusters of four carbon atoms in tetrahedra in space group I43m having four atoms per primitive unit cell (eight per conventional unit cell), and a structure the authors called "carbon sodalite". They found in favor of this carbon sodalite structure, with a calculated density of 2.927 g/cm³, shown in the upper left of the illustration under the abstract.^[29] This structure has just six atoms per primitive unit cell (twelve per conventional unit cell). The carbon atoms are in the same locations as the silicon and aluminum atoms of the mineral sodalite. The space group, I43m, is the same as the fully expanded form of sodalite would have if sodalite had just silicon or just aluminum.^[30]

• bct-carbon: Body-centered tetragonal carbon was proposed by theorists in 2010.[31]^[32]
• Chaoite is a mineral believed to have been formed in meteorite impacts. It has been described as slightly harder than graphite with a reflection color of grey to white. However, the existence of carbyne phases is disputed – see the article on chaoite for details.
• D-carbon: D-carbon was proposed by theorists in 2018.^[33] D-carbon is an orthorhombic sp³ carbon allotrope (6 atoms per cell). Total-energy calculations demonstrate that D-carbon is energetically more favorable than the previously proposed T₆ structure (with 6 atoms per cell) as well as many others.
• Haeckelites: Ordered arrangements of pentagons, hexagons, and heptagons which can either be flat or tubular.

• The Laves graph or K₄ crystal is a theoretically predicted three-dimensional crystalline metastable carbon structure in which each carbon atom is bonded to three others, at 120° angles (like graphite), but where the bond planes of adjacent layers lie at an angle of 70.5°, rather than coinciding.^[34][35]
• M-carbon: Monoclinic C-centered carbon is thought to have been first created in 1963 by compressing graphite at room temperature. Its structure was theorized in 2006,^[36] then in 2009 it was related to those experimental observations.^[37] Many structural candidates, including bct-carbon, were proposed to be equally compatible with experimental data available at the time, until in 2012 it was shown theoretically that this structure is kinetically the most likely to form from graphite.^[38][39] High-resolution data appeared shortly after, demonstrating that among all structure candidates only M-carbon is compatible with experiment.^[40][41]
• Metallic carbon: Theoretical studies have shown that there are regions in the phase diagram, at extremely high pressures, where carbon has metallic character.^[42] Laser shock experiments and theory indicate that above 600 GPa liquid carbon is metallic.^[43]
• Novamene: A combination of both hexagonal diamond and sp² hexagons as in graphene.^[44]
• Phagraphene: Graphene-like allotrope with distorted Dirac cones.
• Prismane C₈ is a theoretically predicted metastable carbon
  triangular prism with two more atoms above and below its bases.^[45]
• Protomene: A hexagonal crystal structure with a fully relaxed primitive cell involving 48 atoms. Out of these, 12 atoms have the potential to switch hybridization between sp² and sp³, forming dimers.^[46]
• Ferromagnetic carbon was discovered in 2015.^[47]
• T-carbon: Every carbon atom in diamond is replaced with a carbon tetrahedron (hence 'T-carbon'). This was proposed by theorists in 1985.[48]
• There is evidence that
  Harvard-Smithsonian Center for Astrophysics described the 2,500-mile (4,000 km)-wide stellar core as a diamond,^[49] and it was named as Lucy, after the Beatles' song "Lucy in the Sky With Diamonds";^[50] however, it is more likely an exotic form of carbon. Penta-graphene
  is a predicted carbon allotrope that utilizes the Cairo pentagonal tiling.
• U carbon is predicted to consist of corrugated layers tiled with six- or 12-atom rings, linked by covalent bonds. Notably, it can be harder than
  permanent magnet at temperatures up to 125 °C.^[51]

• Zayedene: A combination of linear sp carbon chains and sp3 bulk carbon. The structure of these crystalline carbon allotropes consists of sp chains inserted in cylindrical cavities periodically arranged in hexagonal diamond (lonsdaleite).^[52][53]

Variability of carbon

The system of carbon allotropes spans an astounding range of extremes, considering that they are all merely structural formations of the same element.

Between diamond and graphite:

• Diamond crystallizes in the
  hexagonal system
  .
• Diamond is clear and transparent, but graphite is black and opaque.
• Diamond is the hardest mineral known (10 on the
  Mohs scale
  ).
• Diamond is the ultimate abrasive, but graphite is soft and is a very good lubricant.
• Diamond is an excellent electrical insulator, but graphite is an excellent conductor.
• Diamond is an excellent thermal conductor, but some forms of graphite are used for thermal insulation (for example heat shields and firebreaks).
• At standard temperature and pressure, graphite is the thermodynamically stable form. Thus diamonds do not exist forever. The conversion from diamond to graphite, however, has a very high activation energy and is therefore extremely slow.

Despite the hardness of diamonds, the chemical bonds that hold the carbon atoms in diamonds together are actually weaker than those that hold together graphite. The difference is that in diamond, the bonds form an inflexible three-dimensional lattice. In graphite, the atoms are tightly bonded into sheets, but the sheets can slide easily over each other, making graphite soft.^[54]

See also

• Superdense carbon allotropes

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External links

• "SACADA: Samara Carbon Allotrope Database". sctms.ru.

• Falcao, Eduardo H.L.; Wudl, Fred (2007). "Carbon allotropes: Beyond graphite and diamond". Journal of Chemical Technology & Biotechnology. 82 (6): 524–531.
  ISSN 0268-2575
  .

• "The Mysterious Allotropes of Carbon". dendritics.com.