In
chemistryChemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
, a
carbide is a compound composed of
carbonCarbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
and a less
electronegativeElectronegativity, symbol χ , is a chemical property that describes the tendency of an atom or a functional group to attract electrons towards itself. An atom's electronegativity is affected by both its atomic number and the distance that its valence electrons reside from the charged nucleus...
element. Carbides can be generally classified by chemical bonding type as follows: (i) salt-like, (ii) covalent compounds, (iii)
interstitial compoundAn interstitial compound or interstitial alloy is a compound that is formed when an atom of sufficiently small radius sits in an interstitial “hole” in a metal lattice. Examples of small atoms are hydrogen, boron, carbon and nitrogen...
s, and (iv) "intermediate"
transition metalThe term transition metal has two possible meanings:*The IUPAC definition states that a transition metal is "an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell." Group 12 elements are not transition metals in this definition.*Some...
carbides. Examples include
calcium carbidethumb|right|Calcium carbide.Calcium carbide is a chemical compound with the chemical formula of CaC2. The pure material is colorless, however pieces of technical grade calcium carbide are grey or brown and consist of only 80-85% of CaC2 . Because of presence of PH3, NH3, and H2S it has a...
,
silicon carbideSilicon carbide , also known as carborundum, is a compound of silicon and carbon with chemical formula SiC. It occurs in nature as the extremely rare mineral moissanite. Silicon carbide powder has been mass-produced since 1893 for use as an abrasive...
,
tungsten carbideTungsten carbide is an inorganic chemical compound containing equal parts of tungsten and carbon atoms. Colloquially, tungsten carbide is often simply called carbide. In its most basic form, it is a fine gray powder, but it can be pressed and formed into shapes for use in industrial machinery,...
(often called simply
carbide), and
cementiteCementite, also known as iron carbide, is a chemical compound of iron and carbon, 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, brittle material, normally classified as a ceramic in its pure form, though it is more...
, each used in key industrial applications.
Salt-like carbides
Salt-like carbides are composed of highly electropositive elements such as the
alkali metalThe alkali metals are a series of chemical elements in the periodic table. In the modern IUPAC nomenclature, the alkali metals comprise the group 1 elements, along with hydrogen. The alkali metals are lithium , sodium , potassium , rubidium , caesium , and francium...
s, alkaline earths, and group 3 metals including
scandiumScandium is a chemical element with symbol Sc and atomic number 21. A silvery-white metallic transition metal, it has historically been sometimes classified as a rare earth element, together with yttrium and the lanthanoids...
,
yttriumYttrium is a chemical element with symbol Y and atomic number 39. It is a silvery-metallic transition metal chemically similar to the lanthanides and it has often been classified as a "rare earth element". Yttrium is almost always found combined with the lanthanides in rare earth minerals and is...
and
lanthanumLanthanum is a chemical element with the symbol La and atomic number 57.Lanthanum is a silvery white metallic element that belongs to group 3 of the periodic table and is the first element of the lanthanide series. It is found in some rare-earth minerals, usually in combination with cerium and...
.
AluminiumAluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
from group 13 forms
carbidesAluminium carbide, chemical formula Al4C3, is a carbide of aluminium. 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:...
, but
galliumGallium is a chemical element that has the symbol Ga and atomic number 31. Elemental gallium does not occur in nature, but as the gallium salt in trace amounts in bauxite and zinc ores. A soft silvery metallic poor metal, elemental gallium is a brittle solid at low temperatures. As it liquefies...
,
indiumIndium is a chemical element with the symbol In and atomic number 49. This rare, very soft, malleable and easily fusible post-transition metal is chemically similar to gallium and thallium, and shows the intermediate properties between these two...
and
thalliumThallium is a chemical element with the symbol Tl and atomic number 81. This soft gray poor metal resembles tin but discolors when exposed to air. The two chemists William Crookes and Claude-Auguste Lamy discovered thallium independently in 1861 by the newly developed method of flame spectroscopy...
do not. These materials feature isolated carbon centers, often described as "C
4−", in the methanides or methides; two atom units, "C
22−" in the acetylides; and three atom units "C
34−" in the sesquicarbides. The naming of ionic carbides is not consistent and can be quite confusing.
Methanides
Carbides of this class decompose in water producing
methaneMethane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
. Two such examples are
aluminium carbideAluminium carbide, chemical formula Al4C3, is a carbide of aluminium. 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:...
and
beryllium carbideBeryllium carbide, or Be2C, is a metal carbide. Similar to diamond, it is a very hard compound.-Preparation:Beryllium carbide is prepared by heating the elements beryllium and carbon at elevated temperatures...
.
Methanides in general chemical context refers to any compound that hydrolyzes to methane, which might include also salts with hydrogenated anions such as , , and . However, according to IUPAC systematic naming conventions, only the last is properly called "methanide". In theory one can describe compounds that contain the methyl group, with relatively large bond polarity between the carbon and non-hydrogen atom, as salts of this anion; however in truth most such compounds, if not all, are, in fact covalent.
Acetylides
Several carbides are assumed to be salts of the acetylide anion C
22– (also called percarbide), which has a
triple bondA covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....
between the two carbon atoms. Alkali metals, alkaline earth metals, and lanthanoid metals form acetylides, e.g., sodium carbide Na
2C
2, calcium carbide CaC
2, and
LaC2Lanthanum carbide is a chemical compound. One way in which it is manufactured is through a process of diffusion and evaporation of graphite and lanthanum metal in a DC arc furnace...
. Lanthanoids also form acetylides with formula M
2C
3. Metals from group 11 too tend to form acetylides, such as
copper(I) acetylideCopper acetylide, or cuprous acetylide, is an inorganic chemical compound with the formula Cu2C2. It is a heat and shock sensitive high explosive, more sensitive than silver acetylide. It is a metal acetylide. It is similar to silver acetylide and calcium carbide, though it is not called carbide in...
and
silver acetylideSilver acetylide is an inorganic chemical compound with the formula Ag2C2, a metal acetylide. The name is derived from the way it is synthesized, and emphasizes that the compound can be regarded as a salt of the weak acid, acetylene. Since acetylene is called "ethyne" in the modern IUPAC...
. Carbides of the actinide elements, which have stoichiometry MC
2 and M
2C
3, are also described as salt-like derivatives of C
22–.
The C-C triple bond length ranges from 109.2 pm in CaC
2 (similar to ethyne), to 130.3 pm in
LaC2Lanthanum carbide is a chemical compound. One way in which it is manufactured is through a process of diffusion and evaporation of graphite and lanthanum metal in a DC arc furnace...
and 134 pm in
UC2Uranium carbide, a carbide of uranium, is a hard refractory ceramic material. It comes in several stoichiometries , such as uranium monocarbide , uranium sesquicarbide ,...
. The bonding in
LaC2Lanthanum carbide is a chemical compound. One way in which it is manufactured is through a process of diffusion and evaporation of graphite and lanthanum metal in a DC arc furnace...
has been described in terms of La
III with the extra electron delocalised into the antibonding orbital on C
22−, explaining the metallic conduction.
Sesquicarbides
The
polyatomic ionA polyatomic ion, also known as a molecular ion, is a charged species composed of two or more atoms covalently bonded or of a metal complex that can be considered as acting as a single unit in the context of acid and base chemistry or in the formation of salts. The prefix "poly-" means "many," in...
C
34–, sometimes called
sesquicarbide, is found in
Li4C3Lithium carbide, Li2C2, often known as dilithium acetylide, is a chemical compound of lithium and carbon, an acetylide. It is an intermediate compound produced during radiocarbon dating procedures...
, Mg
2C
3. The ion is linear and is isoelectronic with CO
2. The C-C distance in Mg
2C
3 is 133.2 pm. Mg
2C
3 yields
methylacetyleneMethylacetylene is an alkyne with the chemical formula H3C≡CH. It is a component of MAPP gas along with its isomer 1,2-propadiene , which is commonly used in gas welding...
, CH
3CCH, on hydrolysis which was the first indication that it may contain C
34–.
Covalent carbides
The carbides of silicon and boron are described as "covalent carbides", although virtually all compounds of carbon exhibit some covalent character.
Silicon carbideSilicon carbide , also known as carborundum, is a compound of silicon and carbon with chemical formula SiC. It occurs in nature as the extremely rare mineral moissanite. Silicon carbide powder has been mass-produced since 1893 for use as an abrasive...
has two similar crystalline forms, which are both related to the diamond structure.
Boron carbideBoron carbide is an extremely hard boron–carbon ceramic material used in tank armor, bulletproof vests, and numerous industrial applications...
, B
4C, on the other hand has an unusual structure which includes icosahedral boron units linked by carbon atoms. In this respect
boron carbideBoron carbide is an extremely hard boron–carbon ceramic material used in tank armor, bulletproof vests, and numerous industrial applications...
is similar to the boron rich
borideIn chemistry a boride is a chemical compound between boron and a less electronegative element, for example silicon boride . The borides are a very large group of compounds that are generally high melting and are not ionic in nature. Some borides exhibit very useful physical properties. The term...
s. Both
silicon carbideSilicon carbide , also known as carborundum, is a compound of silicon and carbon with chemical formula SiC. It occurs in nature as the extremely rare mineral moissanite. Silicon carbide powder has been mass-produced since 1893 for use as an abrasive...
, SiC, (carborundum) and
boron carbideBoron carbide is an extremely hard boron–carbon ceramic material used in tank armor, bulletproof vests, and numerous industrial applications...
, B
4C are very hard materials and
refractoryA refractory material is one that retains its strength at high temperatures. ASTM C71 defines refractories as "non-metallic materials having those chemical and physical properties that make them applicable for structures, or as components of systems, that are exposed to environments above...
. Both materials are important industrially. Boron also forms other covalent carbides, e.g. B
25C.
Interstitial carbides
The carbides of the group 4, 5 and 6 transition metals (with the exception of chromium) are often described as
interstitial compoundAn interstitial compound or interstitial alloy is a compound that is formed when an atom of sufficiently small radius sits in an interstitial “hole” in a metal lattice. Examples of small atoms are hydrogen, boron, carbon and nitrogen...
s. These carbides have metallic properties and are
refractoryA refractory material is one that retains its strength at high temperatures. ASTM C71 defines refractories as "non-metallic materials having those chemical and physical properties that make them applicable for structures, or as components of systems, that are exposed to environments above...
. Some exhibit a range of stoichiometries, e.g.
titanium carbideTitanium carbide, TiC, is an extremely hard refractory ceramic material, similar to tungsten carbide.It is commercially used in tool bits. It has the appearance of black powder with NaCl-type face centered cubic crystal structure...
, TiC.
Titanium carbideTitanium carbide, TiC, is an extremely hard refractory ceramic material, similar to tungsten carbide.It is commercially used in tool bits. It has the appearance of black powder with NaCl-type face centered cubic crystal structure...
and
tungsten carbideTungsten carbide is an inorganic chemical compound containing equal parts of tungsten and carbon atoms. Colloquially, tungsten carbide is often simply called carbide. In its most basic form, it is a fine gray powder, but it can be pressed and formed into shapes for use in industrial machinery,...
are important industrially and are used to coat metals in cutting tools.
The longheld 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 . Carl Friedrich Gauss proved that the highest average density – that is, the greatest fraction of space occupied by spheres – that can be achieved by a regular lattice...
, (ccp), then filling all of the octahedral interstices with carbon achieves 1:1 stoichiometry with the rock salt structure, (note that in rock salt, NaCl, it is the chloride anions that are cubic close packedIn geometry, close-packing of equal spheres is a dense arrangement of congruent spheres in an infinite, regular arrangement . Carl Friedrich Gauss proved that the highest average density – that is, the greatest fraction of space occupied by spheres – that can be achieved by a regular lattice...
).
- 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 . Carl Friedrich Gauss proved that the highest average density – that is, the greatest fraction of space occupied by spheres – that can be achieved by a regular lattice...
, (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 actual structures of the metals and their carbides. (N.B. the body centred cubic structure adopted by vanadium, niobium, tantalum, chromium, molybdenum and tungsten is not a close packed lattice.) The notation "h/2" refers to the M
2C 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.
| Metal |
Structure of pure metal |
Metallic
radius (pm) |
MC
metal atom packing |
MC structure |
M2C
metal atom packing |
M2C structure |
Other carbides |
| titanium Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....
|
hcp |
147 |
ccp |
rock salt |
|
|
|
zirconiumZirconium is a chemical element with the symbol Zr and atomic number 40. The name of zirconium is taken from the mineral zircon. Its atomic mass is 91.224. It is a lustrous, grey-white, strong transition metal that resembles titanium...
|
hcp |
160 |
ccp |
rock salt |
|
|
|
hafniumHafnium is a chemical element with the symbol Hf and atomic number 72. A lustrous, silvery gray, tetravalent transition metal, hafnium chemically resembles zirconium and is found in zirconium minerals. Its existence was predicted by Dmitri Mendeleev in 1869. Hafnium was the penultimate stable...
|
hcp |
159 |
ccp |
rock salt |
|
|
|
| vanadium Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery gray, ductile and malleable transition metal. The formation of an oxide layer stabilizes the metal against oxidation. The element is found only in chemically combined form in nature...
|
cubic body centered |
134 |
ccp |
rock salt |
hcp |
h/2 |
V4C3 |
niobiumNiobium or columbium , is a chemical element with the symbol Nb and atomic number 41. It's a soft, grey, ductile transition metal, which is often found in the pyrochlore mineral, the main commercial source for niobium, and columbite...
|
cubic body centered |
146 |
ccp |
rock salt |
hcp |
h/2 |
Nb4C3 |
tantalumTantalum is a chemical element with the symbol Ta and atomic number 73. Previously known as tantalium, the name comes from Tantalus, a character in Greek mythology. Tantalum is a rare, hard, blue-gray, lustrous transition metal that is highly corrosion resistant. It is part of the refractory...
|
cubic body centered |
146 |
ccp |
rock salt |
hcp |
h/2 |
Ta4C3 |
chromiumChromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...
|
cubic body centered |
128 |
|
|
|
|
Cr23C6, Cr3C,
Cr7C3, Cr3C2 |
molybdenumMolybdenum , is a Group 6 chemical element with the symbol Mo and atomic number 42. The name is from Neo-Latin Molybdaenum, from Ancient Greek , meaning lead, itself proposed as a loanword from Anatolian Luvian and Lydian languages, since its ores were confused with lead ores...
|
cubic body centered |
139 |
|
hexagonal |
hcp |
h/2 |
Mo3C2 |
tungstenTungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...
|
cubic body centered |
139 |
|
hexagonal |
hcp |
h/2 |
|
For a long time the non-stoichiometric phases were believed to be disordered with a random filling of the interstices, however short and longer range ordering has been detected.
Intermediate transition metal carbides
In these carbides, the transition metal ion is smaller than the critical 135 pm, and the structures are not interstitial but are more complex. Multiple stoichiometries are common, for example iron forms a number of carbides, Fe
3C, Fe
7C
3 and Fe
2C. The best known is
cementiteCementite, also known as iron carbide, is a chemical compound of iron and carbon, 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, brittle material, normally classified as a ceramic in its pure form, though it is more...
, Fe
3C, 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 all are 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.
Molecular carbides
Metal complexes containing C
n fragments are well known. Most common are carbon-centered clusters, such as [Au
6C(PPh
3)
6]
2+. Similar species are known for the
metal carbonylMetal carbonyls are coordination complexes of transition metals with carbon monoxide ligands. These complexes may be homoleptic, that is containing only CO ligands, such as nickel carbonyl , but more commonly metal carbonyls contain a mix of ligands, such as Re3Cl...
s and the early metal halides. Even terminal carbides have been crystallized, e.g., CRuCl
2(P(C
6H
11)
3)
2.
Impossible carbides
Some metals, such as
leadLead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
and
tinTin is a chemical element with the symbol Sn and atomic number 50. It is a main group metal in group 14 of the periodic table. Tin shows chemical similarity to both neighboring group 14 elements, germanium and lead and has two possible oxidation states, +2 and the slightly more stable +4...
, are believed not to form carbides under any circumstances. There exists however a mixed titanium-tin carbide, which is a two-dimensional conductor. (In 2007, there were two reports of a
lead carbide-Production:Lead and carbon do not normally combine, even at very high temperatures. Modern literature on lead carbide is almost non-existent.J. F. Durand reported in 1923 the synthesis of lead carbide from calcium carbide CaC2 by treatment with an aqueous solution of lead acetate Pb2, but this...
PbC
2, apparently of the acetylide type; but these claims have yet to be published in reviewed journals.)
Related materials
In addition to the carbides, other groups of related carbon compounds exist:
- graphite intercalation compound
Graphite intercalation compounds are complex materials having formula XCy where element or molecule X is inserted between the graphite layers. In this type of compound, the graphite layers remain largely intact and the guest molecules or atoms are located in between...
s
- alkali metal fullerides
- endohedral fullerenes
Endohedral fullerenes are fullerenes that have additional atoms, ions, or clusters enclosed within their inner spheres. The first lanthanum C60 complex was synthesized in 1985 called La@C60. The @ sign in the name reflects the notion of a small molecule trapped inside a shell...
, where the metal atom is encapsulated inside a fullerene molecule
- metallacarbohedrenes(met-cars) which are cluster compounds containing C2 units.
- tunable nanoporous carbon
Ultracapacitors may have the potential to become key components for energy storage in the industrial market with the rising push for environmental technology. There are several different approaches to creating ultracapacitors, as detailed here, and tunable nanoporous carbon is a relatively new...
, 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 complex
A transition metal carbene complex is a organometallic compound featuring a divalent organic ligand. The divalent organic ligand coordinated to the metal center is called a carbene. Carbene complexes for almost all transition metals have been reported. Many methods for synthesizing them and...
es.