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In
chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds made of atoms, molecules and ions ...
, a carbide usually describes a compound composed of
carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...
and a metal. In
metallurgy Metallurgy is a domain of materials science and engineering that studies the physical and chemical behavior of metallic elements, their inter-metallic compounds, and their mixtures, which are known as alloys. Metallurgy encompasses both the sc ...
, carbiding or carburizing is the process for producing carbide coatings on a metal piece.


Interstitial / Metallic carbides

The carbides of the group 4, 5 and 6 transition metals (with the exception of chromium) are often described as
interstitial compound In materials science, an interstitial defect is a type of point crystallographic defect where an atom of the same or of a different type, occupies an interstitial site in the crystal structure. When the atom is of the same type as those already ...
s. These carbides have metallic properties and are
refractory In materials science, a refractory material or refractory is a material that is resistant to decomposition by heat, pressure, or chemical attack, and retains strength and form at high temperatures. Refractories are polycrystalline, polyphase, ...
. Some exhibit a range of
stoichiometries Stoichiometry refers to the relationship between the quantities of reactants and products before, during, and following chemical reactions. Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equals ...
, being a non-stoichiometric mixture of various carbides arising due to crystal defects. Some of them, including titanium carbide and
tungsten carbide Tungsten carbide (chemical formula: WC) is a chemical compound (specifically, a carbide) containing equal parts of tungsten and carbon atoms. In its most basic form, tungsten carbide is a fine gray powder, but it can be pressed and formed into ...
, are important industrially and are used to coat metals in cutting tools. The long-held view is that the carbon atoms fit into octahedral interstices in a close-packed metal lattice when the metal atom radius is greater than approximately 135 pm: *When the metal atoms are
cubic close-packed In geometry, close-packing of equal spheres is a dense arrangement of congruent spheres in an infinite, regular arrangement (or lattice). Carl Friedrich Gauss proved that the highest average density – that is, the greatest fraction of space occu ...
, (ccp), then filling all of the octahedral interstices with carbon achieves 1:1 stoichiometry with the rock salt structure. *When the metal atoms are
hexagonal close-packed In geometry, close-packing of equal spheres is a dense arrangement of congruent spheres in an infinite, regular arrangement (or lattice). Carl Friedrich Gauss proved that the highest average density – that is, the greatest fraction of space occu ...
, (hcp), as the octahedral interstices lie directly opposite each other on either side of the layer of metal atoms, filling only one of these with carbon achieves 2:1 stoichiometry with the CdI2 structure. The following table shows structures of the metals and their carbides. (N.B. the body centered cubic structure adopted by vanadium, niobium, tantalum, chromium, molybdenum and tungsten is not a close-packed lattice.) The notation "h/2" refers to the M2C type structure described above, which is only an approximate description of the actual structures. The simple view that the lattice of the pure metal "absorbs" carbon atoms can be seen to be untrue as the packing of the metal atom lattice in the carbides is different from the packing in the pure metal, although it is technically correct that the carbon atoms fit into the octahedral interstices of a close-packed metal lattice. For a long time the
non-stoichiometric In chemistry, non-stoichiometric compounds are chemical compounds, almost always solid inorganic compounds, having elemental composition whose proportions cannot be represented by a ratio of small natural numbers (i.e. an empirical formula); mos ...
phases were believed to be disordered with a random filling of the interstices, however short and longer range ordering has been detected. Iron forms a number of carbides, , and . The best known is
cementite Cementite (or iron carbide) is a compound of iron and carbon, more precisely an intermediate transition metal carbide with the formula Fe3C. By weight, it is 6.67% carbon and 93.3% iron. It has an orthorhombic crystal structure. It is a hard, brit ...
, Fe3C, which is present in steels. These carbides are more reactive than the interstitial carbides; for example, the carbides of Cr, Mn, Fe, Co and Ni are all hydrolysed by dilute acids and sometimes by water, to give a mixture of hydrogen and hydrocarbons. These compounds share features with both the inert interstitials and the more reactive salt-like carbides. Some metals, such as
lead Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...
and
tin Tin is a chemical element with the symbol Sn (from la, stannum) and atomic number 50. Tin is a silvery-coloured metal. Tin is soft enough to be cut with little force and a bar of tin can be bent by hand with little effort. When bent, t ...
, are believed not to form carbides under any circumstances. There exists however a mixed titanium-tin carbide, which is a two-dimensional conductor.


Chemical classification of carbides

Carbides can be generally classified by the chemical bonds type as follows: # salt-like (ionic), # covalent compounds, #
interstitial compound In materials science, an interstitial defect is a type of point crystallographic defect where an atom of the same or of a different type, occupies an interstitial site in the crystal structure. When the atom is of the same type as those already ...
s, and # "intermediate"
transition metal In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that can ...
carbides. Examples include calcium carbide (CaC2), silicon carbide (SiC),
tungsten carbide Tungsten carbide (chemical formula: WC) is a chemical compound (specifically, a carbide) containing equal parts of tungsten and carbon atoms. In its most basic form, tungsten carbide is a fine gray powder, but it can be pressed and formed into ...
(WC; often called, simply, ''carbide'' when referring to machine tooling), and
cementite Cementite (or iron carbide) is a compound of iron and carbon, more precisely an intermediate transition metal carbide with the formula Fe3C. By weight, it is 6.67% carbon and 93.3% iron. It has an orthorhombic crystal structure. It is a hard, brit ...
(Fe3C), each used in key industrial applications. The naming of ionic carbides is not systematic.


Salt-like / saline / ionic carbides

Salt-like carbides are composed of highly electropositive elements such as the
alkali metal The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K),The symbols Na and K for sodium and potassium are derived from their Latin names, ''natrium'' and ''kalium''; these are still the origins of the names ...
s, alkaline earth metals,
lanthanide The lanthanide () or lanthanoid () series of chemical elements comprises the 15 metallic chemical elements with atomic numbers 57–71, from lanthanum through lutetium. These elements, along with the chemically similar elements scandium and yttr ...
s,
actinide The actinide () or actinoid () series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium. The actinide series derives its name from the first element in the series, actinium. The inform ...
s, and group 3 metals (
scandium Scandium is a chemical element with the symbol Sc and atomic number 21. It is a silvery-white metallic d-block element. Historically, it has been classified as a rare-earth element, together with yttrium and the Lanthanides. It was discovered in ...
, yttrium, and lutetium).
Aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...
from group 13 forms carbides, but
gallium Gallium is a chemical element with the symbol Ga and atomic number 31. Discovered by French chemist Paul-Émile Lecoq de Boisbaudran in 1875, Gallium is in group 13 of the periodic table and is similar to the other metals of the group (aluminiu ...
, indium, and
thallium Thallium is a chemical element with the Symbol (chemistry), symbol Tl and atomic number 81. It is a gray post-transition metal that is not found free in nature. When isolated, thallium resembles tin, but discolors when exposed to air. Chemists W ...
do not. These materials feature isolated carbon centers, often described as "C4−", in the methanides or methides; two-atom units, "", in the acetylides; and three-atom units, "", in the allylides. The graphite intercalation compound KC8, prepared from vapour of potassium and graphite, and the alkali metal derivatives of C60 are not usually classified as carbides.


Methanides

Methanides are a subset of carbides distinguished by their tendency to decompose in water producing
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Eart ...
. Three examples are
aluminium carbide Aluminum carbide, chemical formula Al4 C3, is a carbide of aluminum. It has the appearance of pale yellow to brown crystals. It is stable up to 1400 °C. It decomposes in water with the production of methane. Structure Aluminum carbide has ...
, magnesium carbide and
beryllium carbide Beryllium carbide, or Be2C, is a metal carbide. Similar to diamond, it is a very hard compound.Beryllium Carbide Info ...
. Transition metal carbides are not saline: their reaction with water is very slow and is usually neglected. For example, depending on surface porosity, 5–30 atomic layers of titanium carbide are hydrolyzed, forming
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Eart ...
within 5 minutes at ambient conditions, following by saturation of the reaction. Note that methanide in this context is a trivial historical name. According to the IUPAC systematic naming conventions, a compound such as NaCH3 would be termed a "methanide", although this compound is often called methylsodium.


Acetylides/ethynides

Several carbides are assumed to be salts of the acetylide anion (also called percarbide, by analogy with
peroxide In chemistry, peroxides are a group of compounds with the structure , where R = any element. The group in a peroxide is called the peroxide group or peroxo group. The nomenclature is somewhat variable. The most common peroxide is hydrogen p ...
), which has a
triple bond A triple bond in chemistry is a chemical bond between two atoms involving six bonding electrons instead of the usual two in a covalent single bond. Triple bonds are stronger than the equivalent single bonds or double bonds, with a bond order o ...
between the two carbon atoms. Alkali metals, alkaline earth metals, and lanthanoid metals form acetylides, for example, sodium carbide Na2C2, calcium carbide CaC2, and LaC2. Lanthanides also form carbides (sesquicarbides, see below) with formula M2C3. Metals from group 11 also tend to form acetylides, such as copper(I) acetylide and
silver acetylide Silver acetylide is an inorganic chemical compound with the formula Ag2C2, a metal acetylide. The compound can be regarded as a salt of the weak acid, acetylene. The salt's anion consists of two carbon atoms linked by a triple bond. The alternate ...
. Carbides of the
actinide elements The actinide () or actinoid () series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium. The actinide series derives its name from the first element in the series, actinium. The inform ...
, which have stoichiometry MC2 and M2C3, are also described as salt-like derivatives of . The C–C triple bond length ranges from 119.2 pm in CaC2 (similar to ethyne), to 130.3 pm in LaC2 and 134 pm in UC2. The bonding in LaC2 has been described in terms of LaIII with the extra electron delocalised into the antibonding orbital on , explaining the metallic conduction.


Allylides

The
polyatomic ion A polyatomic ion, also known as a molecular ion, is a covalent bonded set of two or more atoms, or of a metal complex, that can be considered to behave as a single unit and that has a net charge that is not zero. The term molecule may or may not ...
, sometimes called allylide, is found in and . The ion is linear and is isoelectronic with . The C–C distance in Mg2C3 is 133.2 pm. yields methylacetylene, CH3CCH, and
propadiene Propadiene () or allene () is the organic compound with the formula . It is the simplest allene, i.e. a compound with two adjacent carbon double bonds. As a constituent of MAPP gas, it has been used as a fuel for specialized welding. Production ...
, CH2CCH2, on hydrolysis, which was the first indication that it contains .


Covalent carbides

The carbides of silicon and
boron Boron is a chemical element with the symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the ''boron group'' it has th ...
are described as "covalent carbides", although virtually all compounds of carbon exhibit some covalent character. Silicon carbide has two similar crystalline forms, which are both related to the diamond structure.
Boron carbide Boron carbide (chemical formula approximately B4C) is an extremely hard boron–carbon ceramic, a covalent material used in tank armor, bulletproof vests, engine sabotage powders, as well as numerous industrial applications. With a Vickers hard ...
, B4C, on the other hand, has an unusual structure which includes icosahedral boron units linked by carbon atoms. In this respect
boron carbide Boron carbide (chemical formula approximately B4C) is an extremely hard boron–carbon ceramic, a covalent material used in tank armor, bulletproof vests, engine sabotage powders, as well as numerous industrial applications. With a Vickers hard ...
is similar to the boron rich borides. Both silicon carbide (also known as ''carborundum'') and boron carbide are very hard materials and
refractory In materials science, a refractory material or refractory is a material that is resistant to decomposition by heat, pressure, or chemical attack, and retains strength and form at high temperatures. Refractories are polycrystalline, polyphase, ...
. Both materials are important industrially. Boron also forms other covalent carbides, such as B25C.


Molecular carbides

Metal complexes containing C are known as
metal carbido complex A metal carbido complex is a coordination complex that contains a carbon atom as a ligand. Carbido complexes are a molecular subclass of carbides, which are prevalent. Carbido complexes represent models for intermediates in Fischer–Tropsch synthe ...
es. Most common are carbon-centered octahedral clusters, such as (where "Ph" represents a
phenyl group In organic chemistry, the phenyl group, or phenyl ring, is a cyclic group of atoms with the formula C6 H5, and is often represented by the symbol Ph. Phenyl group is closely related to benzene and can be viewed as a benzene ring, minus a hydrogen ...
) and e6C(CO)6sup>2−. Similar species are known for the metal carbonyls and the early metal halides. A few terminal carbides have been isolated, such as . Metallocarbohedrynes (or "met-cars") are stable clusters with the general formula where M is a transition metal (Ti, Zr, V, etc.).


Related materials

In addition to the carbides, other groups of related carbon compounds exist: *
graphite intercalation compound Graphite intercalation compounds are complex materials having a formula where the ion or is inserted ( intercalated) between the oppositely charged carbon layers. Typically ''m'' is much less than 1. These materials are deeply colored solids t ...
s *alkali metal
fullerides Fullerides are chemical compounds containing fullerene anions. Common fullerides are derivatives of the most common fullerenes, i.e. C60 and C70. The scope of the area is large because multiple charges are possible, i.e., ''n''− (''n'' = 1, ...
*
endohedral fullerenes Endohedral fullerenes, also called endofullerenes, are fullerenes that have additional atoms, ions, or clusters enclosed within their inner spheres. The first lanthanum C60 complex called La@C60 was synthesized in 1985. The @ (at sign) in the n ...
, where the metal atom is encapsulated within a fullerene molecule *metallacarbohedrenes (met-cars) which are cluster compounds containing C2 units. *
tunable nanoporous carbon Carbide-derived carbon (CDC), also known as tunable nanoporous carbon, is the common term for carbon materials derived from carbide precursors, such as binary (e.g. SiC, TiC), or ternary carbides, also known as MAX phases (e.g., Ti2AlC, Ti3SiC2). CD ...
, where gas chlorination of metallic carbides removes metal molecules to form a highly porous, near-pure carbon material capable of high-density energy storage. * transition metal carbene complexes. * two-dimensional transition metal carbides:
MXenes In materials science, MXenes are a class of two-dimensional inorganic compounds , that consist of atomically thin layers of transition metal carbides, nitrides, or carbonitrides. MXenes accept a variety of hydrophilic terminations. MXenes were r ...


See also

*
Kappa-carbides κ-Carbides are a special class of carbide structures. They are most known for appearing in steels containing manganese and aluminium where they have the molecular formula . Properties Structure κ-Carbides crystallise in the perovskite st ...


References

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